Summa Th. II-II EN Qu.94 a.2

Whether idolatry is a sin?

Objection: 1. It would seem that idolatry is not a sin. Nothing is a sin that the true faith employs in worshipping God. Now the true faith employs images for the divine worship: since both in the Tabernacle were there images of the cherubim, as related in Ex 25, and in the Church are images set up which the faithful worship. Therefore idolatry, whereby idols are worshipped, is not a sin.
2. Further, reverence should be paid to every superior. But the angels and the souls of the blessed are our superiors. Therefore it will be no sin to pay them reverence by worship, of sacrifices or the like.
3. Further, the most high God should be honored with an inward worship, according to Jn 4,24, "God . . . they must adore . . . in spirit and in truth": and Augustine says (Enchiridion iii), that "God is worshipped by faith, hope and charity." Now a man may happen to worship idols outwardly, and yet not wander from the true faith inwardly. Therefore it seems that we may worship idols outwardly without prejudice to the divine worship.

On the contrary It is written (Ex 20,5): "Thou shalt not adore them," i.e. outwardly, "nor serve them," i.e. inwardly, as a gloss explains it: and it is a question of graven things and images. Therefore it is a sin to worship idols whether outwardly or inwardly.
I answer that There has been a twofold error in this matter. For some [*The School of Plato] have thought that to offer sacrifices and other things pertaining to latria, not only to God but also to the others aforesaid, is due and good in itself, since they held that divine honor should be paid to every superior nature, as being nearer to God. But this is unreasonable. For though we ought to revere all superiors, yet the same reverence is not due to them all: and something special is due to the most high God Who excels all in a singular manner: and this is the worship of latria.Nor can it be said, as some have maintained, that "these visible sacrifices are fitting with regard to other gods, and that to the most high God, as being better than those others, better sacrifices, namely, the service of a pure mind, should be offered" [*Augustine, as quoted below]. The reason is that, as Augustine says (De Civ. Dei x, 19), "external sacrifices are signs of internal, just as audible words are signs of things. Wherefore, just as by prayer and praise we utter significant words to Him, and offer to Him in our hearts the things they signify, so too in our sacrifices we ought to realize that we should offer a visible sacrifice to no other than to Him Whose invisible sacrifice we ourselves should be in our hearts."Others held that the outward worship of latria should be given to idols, not as though it were something good or fitting in itself, but as being in harmony with the general custom. Thus Augustine (De Civ. Dei vi, 10) quotes Seneca as saying: "We shall adore," says he, "in such a way as to remember that our worship ss in accordance with custom rather than with the reality": and (De Vera Relig. v) Augustine says that "we must not seek religion from the philosophers, who accepted the same things for sacred, as did the people; and gave utterance in the schools to various and contrary opinions about the nature of their gods, and the sovereign good." This error was embraced also by certain heretics [*The Helcesaitae], who affirmed that it is not wrong for one who is seized in time of persecution to worship idols outwardly so long as he keeps the faith in his heart.But this is evidently false. For since outward worship is a sign of the inward worship, just as it is a wicked lie to affirm the contrary of what one holds inwardly of the true faith so too is it a wicked falsehood to pay outward worship to anything counter to the sentiments of one's heart. Wherefore Augustine condemns Seneca (De Civ. Dei vi, 10) in that "his worship of idols was so much the more infamous forasmuch as the things he did dishonestly were so done by him that the people believed him to act honestly."

Reply to Objection: 1. Neither in the Tabernacle or Temple of the Old Law, nor again now in the Church are images set up that the worship of latria may be paid to them, but for the purpose of signification, in order that belief in the excellence of angels and saints may be impressed and confirmed in the mind of man. It is different with the image of Christ, to which latria is due on account of His Divinity, as we shall state in the III 25,3.
2. 3. The Replies to the Second and Third Objections are evident from what has been said above.

Whether idolatry is the gravest of sins?

Objection: 1. It would seem that idolatry is not the gravest of sins. The worst is opposed to the best (Ethic. viii, 10). But interior worship, which consists of faith, hope and charity, is better than external worship. Therefore unbelief, despair and hatred of God, which are opposed to internal worship, are graver sins than idolatry, which is opposed to external worship.
2. Further, the more a sin is against God the more grievous it is. Now, seemingly, a man acts more directly against God by blaspheming, or denying the faith, than by giving God's worship to another, which pertains to idolatry. Therefore blasphemy and denial of the faith are more grievous sins than idolatry.
3. Further, it seems that lesser evils are punished with greater evils. But the sin of idolatry was punished with the sin against nature, as stated in Rm 1,26. Therefore the sin against nature is a graver sin than idolatry.
4. Further, Augustine says (Contra Faust. xx, 5): "Neither do we say that you," viz. the Manichees, "are pagans, or a sect of pagans, but that you bear a certain likeness to them since you worship many gods: and yet you are much worse than they are, for they worship things that exist, but should not be worshiped as gods, whereas you worship things that exist not at all." Therefore the vice of heretical depravity is more grievous than idolatry.
5. Further, a gloss of Jerome on Ga 4,9, "How turn you again to the weak and needy elements?" says: "The observance of the Law, to which they were then addicted, was a sin almost equal to the worship of idols, to which they had been given before their conversion." Therefore idolatry is not the most grievous sin.

On the contrary A gloss on the saying of Lv 15,25, about the uncleanness of a woman suffering from an issue of blood, says: "Every sin is an uncleanness of the soul, but especially idolatry."
I answer that The gravity of a sin may be considered in two ways. First, on the part of the sin itself, and thus idolatry is the most grievous sin. For just as the most heinous crime in an earthly commonwealth would seem to be for a man to give royal honor to another than the true king, since, so far as he is concerned, he disturbs the whole order of the commonwealth, so, in sins that are committed against God, which indeed are the greater sins, the greatest of all seems to be for a man to give God's honor to a creature, since, so far as he is concerned, he sets up another God in the world, and lessens the divine sovereignty. Secondly, the gravity of a sin may be considered on the part of the sinner. Thus the sin of one that sins knowingly is said to be graver than the sin of one that sins through ignorance: and in this way nothing hinders heretics, if they knowingly corrupt the faith which they have received, from sinning more grievously than idolaters who sin through ignorance. Furthermore other sins may be more grievous on account of greater contempt on the part of the sinner.

Reply to Objection: 1. Idolatry presupposes internal unbelief, and to this it adds undue worship. But in a case of external idolatry without internal unbelief, there is an additional sin of falsehood, as stated above (Article [2]).
2. Idolatry includes a grievous blasphemy, inasmuch as it deprives God of the singleness of His dominion and denies the faith by deeds.
3. Since it is essential to punishment that it be against the will, a sin whereby another sin is punished needs to be more manifest, in order that it may make the man more hateful to himself and to others; but it need not be a more grievous sin: and in this way the sin against nature is less grievous than the sin of idolatry. But since it is more manifest, it is assigned as a fitting punishment of the sin of idolatry, in order that, as by idolatry man abuses the order of the divine honor, so by the sin against nature he may suffer confusion from the abuse of his own nature.
4. Even as to the genus of the sin, the Manichean heresy is more grievous than the sin of other idolaters, because it is more derogatory to the divine honor, since they set up two gods in opposition to one another, and hold many vain and fabulous fancies about God. It is different with other heretics, who confess their belief in one God and worship Him alone.
5. The observance of the Law during the time of grace is not quite equal to idolatry as to the genus of the sin, but almost equal, because both are species of pestiferous superstition.

Whether the cause of idolatry was on the part of man?

Objection: 1. It would seem that the cause of idolatry was not on the part of man. In man there is nothing but either nature, virtue, or guilt. But the cause of idolatry could not be on the part of man's nature, since rather does man's natural reason dictate that there is one God, and that divine worship should not be paid to the dead or to inanimate beings. Likewise, neither could idolatry have its cause in man on the part of virtue, since "a good tree cannot bring forth evil fruit," according to Mt 7,18: nor again could it be on the part of guilt, because, according to Sg 14,27, "the worship of abominable idols is the cause and the beginning and end of all evil." Therefore idolatry has no cause on the part of man.
2. Further, those things which have a cause in man are found among men at all times. Now idolatry was not always, but is stated [*Peter Comestor, Hist. Genes. xxxvii, xl] to have been originated either by Nimrod, who is related to have forced men to worship fire, or by Ninus, who caused the statue of his father Bel to be worshiped. Among the Greeks, as related by Isidore (Etym. viii, 11), Prometheus was the first to set up statues of men: and the Jews say that Ismael was the first to make idols of clay. Moreover, idolatry ceased to a great extent in the sixth age. Therefore idolatry had no cause on the part of man.
3. Further, Augustine says (De Civ. Dei xxi, 6): "It was not possible to learn, for the first time, except from their" (i.e. the demons') "teaching, what each of them desired or disliked, and by what name to invite or compel him: so as to give birth to the magic arts and their professors": and the same observation seems to apply to idolatry. Therefore idolatry had no cause on the part of man.

On the contrary It is written (Sg 14,14): "By the vanity of men they," i.e. idols, "came into the world."
I answer that Idolatry had a twofold cause. One was a dispositive cause; this was on the part of man, and in three ways. First, on account of his inordinate affections, forasmuch as he gave other men divine honor, through either loving or revering them too much. This cause is assigned (Sg 14,15): "A father being afflicted with bitter grief, made to himself the image of his son, who was quickly taken away: and him who then had died as a man he began to worship as a god." The same passage goes on to say (Sg 14,21) that "men serving either their affection, or their kings, gave the incommunicable name [Vulg.: 'names']," i.e. of the Godhead, "to stones and wood." Secondly, because man takes a natural pleasure in representations, as the Philosopher observes (Poet. iv), wherefore as soon as the uncultured man saw human images skillfully fashioned by the diligence of the craftsman, he gave them divine worship; hence it is written (Sg 13,11-17): "If an artist, a carpenter, hath cut down a tree, proper for his use, in the wood . . . and by the skill of his art fashioneth it, and maketh it like the image of a man . . . and then maketh prayer to it, inquiring concerning his substance, and his children, or his marriage." Thirdly, on account of their ignorance of the true God, inasmuch as through failing to consider His excellence men gave divine worship to certain creatures, on account of their beauty or power, wherefore it is written (Sg 13,1-2): "All men . . . neither by attending to the works have acknowledged who was the workman, but have imagined either the fire, or the wind, or the swift air, or the circle of the stars, or the great water, or the sun and the moon, to be the gods that rule the world."The other cause of idolatry was completive, and this was on the part of the demons, who offered themselves to be worshipped by men, by giving answers in the idols, and doing things which to men seemed marvelous. Hence it is written (Ps 95,5): "All the gods of the Gentiles are devils."

Reply to Objection: 1. The dispositive cause of idolatry was, on the part of man, a defect of nature, either through ignorance in his intellect, or disorder in his affections, as stated above; and this pertains to guilt. Again, idolatry is stated to be the cause, beginning and end of all sin, because there is no kind of sin that idolatry does not produce at some time, either through leading expressly to that sin by causing it, or through being an occasion thereof, either as a beginning or as an end, in so far as certain sins were employed in the worship of idols; such as homicides, mutilations, and so forth. Nevertheless certain sins may precede idolatry and dispose man thereto.
2. There was no idolatry in the first age, owing to the recent remembrance of the creation of the world, so that man still retained in his mind the knowledge of one God. In the sixth age idolatry was banished by the doctrine and power of Christ, who triumphed over the devil.
3. This argument considers the consummative cause of idolatry.


We must now consider superstition in divinations, under which head there are eight points of inquiry:

(1) Whether divination is a sin?

(2) Whether it is a species of superstition?

(3) Of the species of divination;

(4) Of divination by means of demons;

(5) Of divination by the stars;

(6) Of divination by dreams;

(7) Of divination by auguries and like observances;

(8) Of divination by lots.

Whether divination is a sin?

Objection: 1. It would seem that divination is not a sin. Divination is derived from something "divine": and things that are divine pertain to holiness rather than to sin. Therefore it seems that divination is not a sin.
2. Further, Augustine says (De Lib. Arb. i, 1): "Who dares to say that learning is an evil?" and again: "I could nowise admit that intelligence can be an evil." But some arts are divinatory, as the Philosopher states (De Memor. i): and divination itself would seem to pertain to a certain intelligence of the truth. Therefore it seems that divination is not a sin.
3.Further, there is no natural inclination to evil; because nature inclines only to its like. But men by natural inclination seek to foreknow future events; and this belongs to divination. Therefore divination is not a sin.

On the contrary It is written (Dt 18,10-11): "Neither let there be found among you . . . any one that consulteth pythonic spirits, or fortune tellers": and it is stated in the Decretals (26, qu. v, can. Qui divinationes): "Those who seek for divinations shall be liable to a penance of five years' duration, according to the fixed grades of penance."
I answer that Divination denotes a foretelling of the future. The future may be foreknown in two ways: first in its causes, secondly in itself. Now the causes of the future are threefold: for some produce their effects, of necessity and always; and such like future effects can be foreknown and foretold with certainty, from considering their causes, even as astrologers foretell a coming eclipse. Other causes produce their effects, not of necessity and always, but for the most part, yet they rarely fail: and from such like causes their future effects can be foreknown, not indeed with certainty, but by a kind of conjecture, even as astrologers by considering the stars can foreknow and foretell things concerning rains and droughts, and physicians, concerning health and death. Again, other causes, considered in themselves, are indifferent; and this is chiefly the case in the rational powers, which stand in relation to opposites, according to the Philosopher [*Metaph. viii, 2,5,8]. Such like effects, as also those which ensue from natural causes by chance and in the minority of instances, cannot be foreknown from a consideration of their causes, because these causes have no determinate inclination to produce these effects. Consequently such like effects cannot be foreknown unless they be considered in themselves. Now man cannot consider these effects in themselves except when they are present, as when he sees Socrates running or walking: the consideration of such things in themselves before they occur is proper to God, Who alone in His eternity sees the future as though it were present, as stated in the I 14,13; I 57,3; I 86,4. Hence it is written (Is 41,23): "Show the things that are to come hereafter, and we shall know that ye are gods." Therefore if anyone presume to foreknow or foretell such like future things by any means whatever, except by divine revelation, he manifestly usurps what belongs to God. It is for this reason that certain men are called divines: wherefore Isidore says (Etym. viii, 9): "They are called divines, as though they were full of God. For they pretend to be filled with the Godhead, and by a deceitful fraud they forecast the future to men."Accordingly it is not called divination, if a man foretells things that happen of necessity, or in the majority of instances, for the like can be foreknown by human reason: nor again if anyone knows other contingent future things, through divine revelation: for then he does not divine, i.e. cause something divine, but rather receives something divine. Then only is a man said to divine, when he usurps to himself, in an undue manner, the foretelling of future events: and this is manifestly a sin. Consequently divination is always a sin; and for this reason Jerome says in his commentary on Mich. 3:9, seqq. that "divination is always taken in an evil sense."

Reply to Objection: 1. Divination takes its name not from a rightly ordered share of something divine, but from an undue usurpation thereof, as stated above.
2. There are certain arts for the foreknowledge of future events that occur of necessity or frequently, and these do not pertain to divination. But there are no true arts or sciences for the knowledge of other future events, but only vain inventions of the devil's deceit, as Augustine says (De Civ. Dei xxi, 8).
3. Man has a natural inclination to know the future by human means, but not by the undue means of divination.

Whether divination is a species of superstition?

Objection: 1. It would seem that divination is not a species of superstition. The same thing cannot be a species of diverse genera. Now divination is apparently a species of curiosity, according to Augustine (De Vera Relig. xxxviii) [*Cf. De Doctr. Christ. ii, 23,24; De Divin. Daem. 3]. Therefore it is not, seemingly, a species of superstition.
2. Further, just as religion is due worship, so is superstition undue worship. But divination does not seem to pertain to undue worship. Therefore it does not pertain to superstition.
3. Further, superstition is opposed to religion. But in true religion nothing is to be found corresponding as a contrary to divination. Therefore divination is not a species of superstition.

On the contrary Origen says in his Peri Archon [*The quotation is from his sixteenth homily on the Book of Numbers]: "There is an operation of the demons in the administering of foreknowledge, comprised, seemingly, under the head of certain arts exercised by those who have enslaved themselves to the demons, by means of lots, omens, or the observance of shadows. I doubt not that all these things are done by the operation of the demons." Now, according to Augustine (De Doctr. Christ. ii, 20,23), "whatever results from fellowship between demons and men is superstitious." Therefore divination is a species of superstition.
I answer that As stated above (Article [1]; Questions [92],94), superstition denotes undue divine worship. Now a thing pertains to the worship of God in two ways: in one way, it is something offered to God; as a sacrifice, an oblation, or something of the kind: in another way, it is something divine that is assumed, as stated above with regard to an oath (Question [89], Article [4], ad 2). Wherefore superstition includes not only idolatrous sacrifices offered to demons, but also recourse to the help of the demons for the purpose of doing or knowing something. But all divination results from the demons' operation, either because the demons are expressly invoked that the future may be made known, or because the demons thrust themselves into futile searchings of the future, in order to entangle men's minds with vain conceits. Of this kind of vanity it is written (Ps 39,5): "Who hath not regard to vanities and lying follies." Now it is vain to seek knowledge of the future, when one tries to get it from a source whence it cannot be foreknown. Therefore it is manifest that divination is a species of superstition.

Reply to Objection: 1. Divination is a kind of curiosity with regard to the end in view, which is foreknowledge of the future; but it is a kind of superstition as regards the mode of operation.
2. This kind of divination pertains to the worship of the demons, inasmuch as one enters into a compact, tacit or express with the demons.
3. In the New Law man's mind is restrained from solicitude about temporal things: wherefore the New Law contains no institution for the foreknowledge of future events in temporal matters. On the other hand in the Old Law, which contained earthly promises, there were consultations about the future in connection with religious matters. Hence where it is written (Is 8,19): "And when they shall say to you: Seek of pythons and of diviners, who mutter in their enchantments," it is added by way of answer: "Should not the people seek of their God, a vision for the living and the dead? [*Vulg.: 'seek of their God, for the living of the dead?']"In the New Testament, however, there were some possessed of the spirit of prophecy, who foretold many things about future events.

Whether we ought to distinguish several species of divination?

Objection: 1. It would seem that we should not distinguish several species of divination. Where the formality of sin is the same, there are not seemingly several species of sin. Now there is one formality of sin in all divinations, since they consist in entering into compact with the demons in order to know the future. Therefore there are not several species of divination.
2. Further, a human act takes it species from its end, as stated above (I-II 1,3; I-II 18,6). But all divination is directed to one end, namely, the foretelling of the future. Therefore all divinations are of one species.
3. Further, signs do not vary the species of a sin, for whether one detracts by word writing or gestures, it is the same species of sin. Now divinations seem to differ merely according to the various signs whence the foreknowledge of the future is derived. Therefore there are not several species of divination.

On the contrary Isidore enumerates various species of divination (Etym. viii, 9).
I answer that As stated above (Article [2]), all divinations seek to acquire foreknowledge of future events, by means of some counsel and help of a demon, who is either expressly called upon to give his help, or else thrusts himself in secretly, in order to foretell certain future things unknown to men, but known to him in such manners as have been explained in the I 57,3. When demons are expressly invoked, they are wont to foretell the future in many ways. Sometimes they offer themselves to human sight and hearing by mock apparitions in order to foretell the future: and this species is called "prestigiation" because man's eyes are blindfolded [praestringuntur]. Sometimes they make use of dreams, and this is called "divination by dreams": sometimes they employ apparitions or utterances of the dead, and this species is called "necromancy," for as Isidore observes (Etym. viii) in Greek, (nekron) "means dead and (manteia) divination, because after certain incantations and the sprinkling of blood, the dead seem to come to life, to divine and to answer questions." Sometimes they foretell the future through living men, as in the case of those who are possessed: this is divination by "pythons," of whom Isidore says that "pythons are so called from Pythius Apollo, who was said to be the inventor of divination." Sometimes they foretell the future by means of shapes or signs which appear in inanimate beings. If these signs appear in some earthly body such as wood, iron or polished stone, it is called "geomancy," if in water "hydromancy," if in the air "aeromancy," if in fire "pyromancy," if in the entrails of animals sacrificed on the altars of demons, "aruspicy."The divination which is practiced without express invocation of the demons is of two kinds. The first is when, with a view to obtain knowledge of the future, we take observations in the disposition of certain things. If one endeavor to know the future by observing the position and movements of the stars, this belongs to "astrologers," who are also called "genethliacs," because they take note of the days on which people are born. If one observe the movements and cries of birds or of any animals, or the sneezing of men, or the sudden movements of limbs, this belongs in general to "augury," which is so called from the chattering of birds [avium garritu], just as "auspice" is derived from watching birds [avium inspectione]. These are chiefly wont to be observed in birds, the former by the ear, the latter by the eye. If, however, these observations have for their object men's words uttered unintentionally, which someone twist so as to apply to the future that he wishes to foreknow, then it is called an "omen": and as Valerius Maximus [*De Dict. Fact. Memor. i, 5] remarks, "the observing of omens has a touch of religion mingled with it, for it is believed to be founded not on a chance movement, but on divine providence. It was thus that when the Romans were deliberating whether they would change their position, a centurion happened to exclaim at the time: 'Standard-bearer, fix the banner, we had best stand here': and on hearing these words they took them as an omen, and abandoned their intention of advancing further." If, however, the observation regards the dispositions, that occur to the eye, of figures in certain bodies, there will be another species of divination: for the divination that is taken from observing the lines of the hand is called "chiromancy," i.e. divination of the hand (because (cheir) is the Greek for hand): while the divination which is taken from signs appearing in the shoulder-blades of an animal is called "spatulamancy."To this second species of divination, which is without express invocation of the demons, belongs that which is practiced by observing certain things done seriously by men in the research of the occult, whether by drawing lots, which is called "geomancy"; or by observing the shapes resulting from molten lead poured into water; or by observing which of several sheets of paper, with or without writing upon them, a person may happen to draw; or by holding out several unequal sticks and noting who takes the greater or the lesser. or by throwing dice, and observing who throws the highest score; or by observing what catches the eye when one opens a book, all of which are named "sortilege."Accordingly it is clear that there are three kinds of divination. The first is when the demons are invoked openly, this comes under the head of "necromancy"; the second is merely an observation of the disposition or movement of some other being, and this belongs to "augury"; while the third consists in doing something in order to discover the occult; and this belongs to "sortilege." Under each of these many others are contained, as explained above.

Reply to Objection: 1. In all the aforesaid there is the same general, but not the same special, character of sin: for it is much more grievous to invoke the demons than to do things that deserve the demons' interference.
2. Knowledge of the future or of the occult is the ultimate end whence divination takes its general formality. But the various species are distinguished by their proper objects or matters, according as the knowledge of the occult is sought in various things.
3. The things observed by diviners are considered by them, not as signs expressing what they already know, as happens in detraction, but as principles of knowledge. Now it is evident that diversity of principles diversifies the species, even in demonstrative sciences.

Whether divination practiced by invoking the demons is unlawful?

Objection: 1. It would seem that divination practiced by invoking the demons is not unlawful. Christ did nothing unlawful, according to 1P 2,22, "Who did no sin." Yet our Lord asked the demon: "What is thy name?" and the latter replied: "My name is Legion, for we are many" (Mc 5,9). Therefore it seems lawful to question the demons about the occult.
2. Further, the souls of the saints do not encourage those who ask unlawfully. Yet Samuel appeared to Saul when the latter inquired of the woman that had a divining spirit, concerning the issue of the coming war (1S 28,8, sqq.). Therefore the divination that consists in questioning demons is not unlawful.
3. Further, it seems lawful to seek the truth from one who knows, if it be useful to know it. But it is sometimes useful to know what is hidden from us, and can be known through the demons, as in the discovery of thefts. Therefore divination by questioning demons is not unlawful.

On the contrary It is written (Dt 18,10-11): "Neither let there be found among you . . . anyone that consulteth soothsayers . . . nor . . . that consulteth pythonic spirits."
I answer that All divination by invoking demons is unlawful for two reasons. The first is gathered from the principle of divination, which is a compact made expressly with a demon by the very fact of invoking him. This is altogether unlawful; wherefore it is written against certain persons (Is 28,15): "You have said: We have entered into a league with death, and we have made a covenant with hell." And still more grievous would it be if sacrifice were offered or reverence paid to the demon invoked. The second reason is gathered from the result. For the demon who intends man's perdition endeavors, by his answers, even though he sometimes tells the truth, to accustom men to believe him, and so to lead him on to something prejudicial to the salvation of mankind. Hence Athanasius, commenting on the words of Lc 4,35, "He rebuked him, saying: Hold thy peace," says: "Although the demon confessed the truth, Christ put a stop to his speech, lest together with the truth he should publish his wickedness and accustom us to care little for such things, however much he may seem to speak the truth. For it is wicked, while we have the divine Scriptures, to seek knowledge from the demons."

Reply to Objection: 1. According to Bede's commentary on Lc 8,30, "Our Lord inquired, not through ignorance, but in order that the disease, which he tolerated, being made public, the power of the Healer might shine forth more graciously." Now it is one thing to question a demon who comes to us of his own accord (and it is lawful to do so at times for the good of others, especially when he can be compelled, by the power of God, to tell the truth) and another to invoke a demon in order to gain from him knowledge of things hidden from us.
2. According to Augustine (Ad Simplic. ii, 3), "there is nothing absurd in believing that the spirit of the just man, being about to smite the king with the divine sentence, was permitted to appear to him, not by the sway of magic art or power, but by some occult dispensation of which neither the witch nor Saul was aware. Or else the spirit of Samuel was not in reality aroused from his rest, but some phantom or mock apparition formed by the machinations of the devil, and styled by Scripture under the name of Samuel, just as the images of things are wont to be called by the names of those things."
3. No temporal utility can compare with the harm to spiritual health that results from the research of the unknown by invoking the demon.

Summa Th. II-II EN Qu.94 a.2