Summa Th. I-II EN Qu.106 a.3
Objection: 1. It would seem that the New Law should have been given from the beginning of the world. "For there is no respect of persons with God" (Rm 2,11). But "all" men "have sinned and do need the glory of God" (Rm 3,23). Therefore the Law of the Gospel should have been given from the beginning of the world, in order that it might bring succor to all.
2. Further, as men dwell in various places, so do they live in various times. But God, "Who will have all men to be saved" (1Tm 2,4), commanded the Gospel to be preached in all places, as may be seen in the last chapters of Matthew and Mark. Therefore the Law of the Gospel should have been at hand for all times, so as to be given from the beginning of the world.
3. Further, man needs to save his soul, which is for all eternity, more than to save his body, which is a temporal matter. But God provided man from the beginning of the world with things that are necessary for the health of his body, by subjecting to his power whatever was created for the sake of man (Gn 1,26-29). Therefore the New Law also, which is very necessary for the health of the soul, should have been given to man from the beginning of the world.
On the contrary The Apostle says (1Co 15,46): "That was not first which is spiritual, but that which is natural." But the New Law is highly spiritual. Therefore it was not fitting for it to be given from the beginning of the world.
I answer that Three reasons may be assigned why it was not fitting for the New Law to be given from the beginning of the world. The first is because the New Law, as stated above (Article ), consists chiefly in the grace of the Holy Ghost: which it behoved not to be given abundantly until sin, which is an obstacle to grace, had been cast out of man through the accomplishment of his redemption by Christ: wherefore it is written (Jn 7,39): "As yet the Spirit was not given, because Jesus was not yet glorified." This reason the Apostle states clearly (Rm 8,2, seqq.) where, after speaking of "the Law of the Spirit of life," he adds: "God sending His own Son, in the likeness of sinful flesh, of sin* hath condemned sin in the flesh, that the justification of the Law might be fulfilled in us." [*St. Thomas, quoting perhaps from memory, omits the "et" (and), after "sinful flesh." The text quoted should read thus: "in the likeness of sinful flesh, and a sin offering ((peri hamartias)), hath," etc.]A second reason may be taken from the perfection of the New Law. Because a thing is not brought to perfection at once from the outset, but through an orderly succession of time; thus one is at first a boy, and then a man. And this reason is stated by the Apostle (Ga 3,24-25): "The Law was our pedagogue in Christ that we might be justified by faith. But after the faith is come, we are no longer under a pedagogue."The third reason is found in the fact that the New Law is the law of grace: wherefore it behoved man first of all to be left to himself under the state of the Old Law, so that through falling into sin, he might realize his weakness, and acknowledge his need of grace. This reason is set down by the Apostle (Rm 5,20): "The Law entered in, that sin might abound: and when sin abounded grace did more abound."
Reply to Objection: 1. Mankind on account of the sin of our first parents deserved to be deprived of the aid of grace: and so "from whom it is withheld it is justly withheld, and to whom it is given, it is mercifully given," as Augustine states (De Perfect. Justit. iv) [*Cf. Ep. ccvii; De Pecc. Mer. et Rem. ii, 19]. Consequently it does not follow that there is respect of persons with God, from the fact that He did not offer the Law of grace to all from the beginning of the world, which Law was to be published in due course of time, as stated above.
2. The state of mankind does not vary according to diversity of place, but according to succession of time. Hence the New Law avails for all places, but not for all times: although at all times there have been some persons belonging to the New Testament, as stated above (Article , ad 3).
3. Things pertaining to the health of the body are of service to man as regards his nature, which sin does not destroy: whereas things pertaining to the health of the soul are ordained to grace, which is forfeit through sin. Consequently the comparison will not hold.
Objection: 1. It would seem that the New Law will not last until the end of the world. Because, as the Apostle says (1Co 13,10), "when that which is perfect is come, that which is in part shall be done away." But the New Law is "in part," since the Apostle says (1Co 13,9): "We know in part and we prophesy in part." Therefore the New Law is to be done away, and will be succeeded by a more perfect state.
2. Further, Our Lord (Jn 16,13) promised His disciples the knowledge of all truth when the Holy Ghost, the Comforter, should come. But the Church knows not yet all truth in the state of the New Testament. Therefore we must look forward to another state, wherein all truth will be revealed by the Holy Ghost.
3. Further, just as the Father is distinct from the Son and the Son from the Father, so is the Holy Ghost distinct from the Father and the Son. But there was a state corresponding with the Person of the Father, viz. the state of the Old Law, wherein men were intent on begetting children: and likewise there is a state corresponding to the Person of the Son: viz. the state of the New Law, wherein the clergy who are intent on wisdom (which is appropriated to the Son) hold a prominent place. Therefore there will be a third state corresponding to the Holy Ghost, wherein spiritual men will hold the first place.
4. Further, Our Lord said (Mt 24,14): "This Gospel of the kingdom shall be preached in the whole world . . . and then shall the consummation come." But the Gospel of Christ is already preached throughout the whole world: and yet the consummation has not yet come. Therefore the Gospel of Christ is not the Gospel of the kingdom, but another Gospel, that of the Holy Ghost, is to come yet, like unto another Law.
On the contrary Our Lord said (Mt 24,34): "I say to you that this generation shall not pass till all (these) things be done": which passage Chrysostom (Hom. lxxvii) explains as referring to "the generation of those that believe in Christ." Therefore the state of those who believe in Christ will last until the consummation of the world.
I answer that The state of the world may change in two ways. In one way, according to a change of law: and thus no other state will succeed this state of the New Law. Because the state of the New Law succeeded the state of the Old Law, as a more perfect law a less perfect one. Now no state of the present life can be more perfect that the state of the New Law: since nothing can approach nearer to the last end than that which is the immediate cause of our being brought to the last end. But the New Law does this: wherefore the Apostle says (He 10,19-22): "Having therefore, brethren, a confidence in the entering into the Holies by the blood of Christ, a new . . . way which He hath dedicated for us . . . let us draw near." Therefore no state of the present life can be more perfect than that of the New Law, since the nearer a thing is to the last end the more perfect it is.In another way the state of mankind may change according as man stands in relation to one and the same law more or less perfectly. And thus the state of the Old Law underwent frequent changes, since at times the laws were very well kept, and at other times were altogether unheeded. Thus, too, the state of the New Law is subject to change with regard to various places, times, and persons, according as the grace of the Holy Ghost dwells in man more or less perfectly. Nevertheless we are not to look forward to a state wherein man is to possess the grace of the Holy Ghost more perfectly than he has possessed it hitherto, especially the apostles who "received the firstfruits of the Spirit, i.e. sooner and more abundantly than others," as a gloss expounds on Rm 8,23.
Reply to Objection: 1. As Dionysius says (Eccl. Hier. v), there is a threefold state of mankind; the first was under the Old Law; the second is that of the New Law; the third will take place not in this life, but in heaven. But as the first state is figurative and imperfect in comparison with the state of the Gospel; so is the present state figurative and imperfect in comparison with the heavenly state, with the advent of which the present state will be done away as expressed in that very passage (1Co 13,12): "We see now through a glass in a dark manner; but then face to face."
2. As Augustine says (Contra Faust. xix, 31), Montanus and Priscilla pretended that Our Lord's promise to give the Holy Ghost was fulfilled, not in the apostles, but in themselves. In like manner the Manicheans maintained that it was fulfilled in Manes whom they held to be the Paraclete. Hence none of the above received the Acts of the Apostles, where it is clearly shown that the aforesaid promise was fulfilled in the apostles: just as Our Lord promised them a second time (Ac 1,5): "You shall be baptized with the Holy Ghost, not many days hence": which we read as having been fulfilled in Ac 2. However, these foolish notions are refuted by the statement (Jn 7,39) that "as yet the Spirit was not given, because Jesus was not yet glorified"; from which we gather that the Holy Ghost was given as soon as Christ was glorified in His Resurrection and Ascension. Moreover, this puts out of court the senseless idea that the Holy Ghost is to be expected to come at some other time.Now the Holy Ghost taught the apostles all truth in respect of matters necessary for salvation; those things, to wit, that we are bound to believe and to do. But He did not teach them about all future events: for this did not regard them according to Ac 1,7: "It is not for you to know the times or moments which the Father hath put in His own power."
3. The Old Law corresponded not only to the Father, but also to the Son: because Christ was foreshadowed in the Old Law. Hence Our Lord said (Jn 5,46): "If you did believe Moses, you would perhaps believe me also; for he wrote of Me." In like manner the New Law corresponds not only to Christ, but also to the Holy Ghost; according to Rm 8,2: "The Law of the Spirit of life in Christ Jesus," etc. Hence we are not to look forward to another law corresponding to the Holy Ghost.
4. Since Christ said at the very outset of the preaching of the Gospel: "The kingdom of heaven is at hand" (Mt 4,17), it is most absurd to say that the Gospel of Christ is not the Gospel of the kingdom. But the preaching of the Gospel of Christ may be understood in two ways. First, as denoting the spreading abroad of the knowledge of Christ: and thus the Gospel was preached throughout the world even at the time of the apostles, as Chrysostom states (Hom. lxxv in Matth.). And in this sense the words that follow---"and then shall the consummation come," refer to the destruction of Jerusalem, of which He was speaking literally. Secondly, the preaching of the Gospel may be understood as extending throughout the world and producing its full effect, so that, to wit, the Church would be founded in every nation. And in these sense, as Augustine writes to Hesychius (Epist. cxcix), the Gospel is not preached to the whole world yet, but, when it is, the consummation of the world will come.
We must now consider the New Law as compared with the Old: under which head there are four points of inquiry:
(1) Whether the New Law is distinct from the Old Law?
(2) Whether the New Law fulfils the Old?
(3) Whether the New Law is contained in the Old?
(4) Which is the more burdensome, the New or the Old Law?
Objection: 1. It would seem that the New Law is not distinct from the Old. Because both these laws were given to those who believe in God: since "without faith it is impossible to please God," according to He 11,6. But the faith of olden times and of nowadays is the same, as the gloss says on Mt 21,9. Therefore the law is the same also.
2. Further, Augustine says (Contra Adamant. Manich. discip. xvii) that "there is little difference between the Law and Gospel" [*The 'little difference' refers to the Latin words 'timor' and 'amor']---"fear and love." But the New and Old Laws cannot be differentiated in respect of these two things: since even the Old Law comprised precepts of charity: "Thou shalt love thy neighbor" (Lv 19,18), and: "Thou shalt love the Lord thy God" (Dt 6,5). In like manner neither can they differ according to the other difference which Augustine assigns (Contra Faust. iv, 2), viz. that "the Old Testament contained temporal promises, whereas the New Testament contains spiritual and eternal promises": since even the New Testament contains temporal promises, according to Mc 10,30: He shall receive "a hundred times as much . . . in this time, houses and brethren," etc.: while in the Old Testament they hoped in promises spiritual and eternal, according to He 11,16: "But now they desire a better, that is to say, a heavenly country," which is said of the patriarchs. Therefore it seems that the New Law is not distinct from the Old.
3. Further, the Apostle seems to distinguish both laws by calling the Old Law "a law of works," and the New Law "a law of faith" (Rm 3,27). But the Old Law was also a law of faith, according to He 11,39: "All were [Vulg.: 'All these being'] approved by the testimony of faith," which he says of the fathers of the Old Testament. In like manner the New Law is a law of works: since it is written (Mt 5,44): "Do good to them that hate you"; and (Lc 22,19): "Do this for a commemoration of Me." Therefore the New Law is not distinct from the Old.
On the contrary the Apostle says (He 7,12): "The priesthood being translated it is necessary that a translation also be made of the Law." But the priesthood of the New Testament is distinct from that of the Old, as the Apostle shows in the same place. Therefore the Law is also distinct.
I answer that As stated above (Question , Article ; Question , Article ), every law ordains human conduct to some end. Now things ordained to an end may be divided in two ways, considered from the point of view of the end. First, through being ordained to different ends: and this difference will be specific, especially if such ends are proximate. Secondly, by reason of being closely or remotely connected with the end. Thus it is clear that movements differ in species through being directed to different terms: while according as one part of a movement is nearer to the term than another part, the difference of perfect and imperfect movement is assessed.Accordingly then two laws may be distinguished from one another in two ways. First, through being altogether diverse, from the fact that they are ordained to diverse ends: thus a state-law ordained to democratic government, would differ specifically from a law ordained to government by the aristocracy. Secondly, two laws may be distinguished from one another, through one of them being more closely connected with the end, and the other more remotely: thus in one and the same state there is one law enjoined on men of mature age, who can forthwith accomplish that which pertains to the common good; and another law regulating the education of children who need to be taught how they are to achieve manly deeds later on.We must therefore say that, according to the first way, the New Law is not distinct from the Old Law: because they both have the same end, namely, man's subjection to God; and there is but one God of the New and of the Old Testament, according to Rm 3,30: "It is one God that justifieth circumcision by faith, and uncircumcision through faith." According to the second way, the New Law is distinct from the Old Law: because the Old Law is like a pedagogue of children, as the Apostle says (Ga 3,24), whereas the New Law is the law of perfection, since it is the law of charity, of which the Apostle says (Col 3,14) that it is "the bond of perfection."
Reply to Objection: 1. The unity of faith under both Testaments witnesses to the unity of end: for it has been stated above (Question , Article ) that the object of the theological virtues, among which is faith, is the last end. Yet faith had a different state in the Old and in the New Law: since what they believed as future, we believe as fact.
2. All the differences assigned between the Old and New Laws are gathered from their relative perfection and imperfection. For the precepts of every law prescribe acts of virtue. Now the imperfect, who as yet are not possessed of a virtuous habit, are directed in one way to perform virtuous acts, while those who are perfected by the possession of virtuous habits are directed in another way. For those who as yet are not endowed with virtuous habits, are directed to the performance of virtuous acts by reason of some outward cause: for instance, by the threat of punishment, or the promise of some extrinsic rewards, such as honor, riches, or the like. Hence the Old Law, which was given to men who were imperfect, that is, who had not yet received spiritual grace, was called the "law of fear," inasmuch as it induced men to observe its commandments by threatening them with penalties; and is spoken of as containing temporal promises. On the other hand, those who are possessed of virtue, are inclined to do virtuous deeds through love of virtue, not on account of some extrinsic punishment or reward. Hence the New Law which derives its pre-eminence from the spiritual grace instilled into our hearts, is called the "Law of love": and it is described as containing spiritual and eternal promises, which are objects of the virtues, chiefly of charity. Accordingly such persons are inclined of themselves to those objects, not as to something foreign but as to something of their own. For this reason, too, the Old Law is described as "restraining the hand, not the will" [*Peter Lombard, Sent. iii, D, 40]; since when a man refrains from some sins through fear of being punished, his will does not shrink simply from sin, as does the will of a man who refrains from sin through love of righteousness: and hence the New Law, which is the Law of love, is said to restrain the will.Nevertheless there were some in the state of the Old Testament who, having charity and the grace of the Holy Ghost, looked chiefly to spiritual and eternal promises: and in this respect they belonged to the New Law. In like manner in the New Testament there are some carnal men who have not yet attained to the perfection of the New Law; and these it was necessary, even under the New Testament, to lead to virtuous action by the fear of punishment and by temporal promises.But although the Old Law contained precepts of charity, nevertheless it did not confer the Holy Ghost by Whom "charity . . . is spread abroad in our hearts" (Rm 5,5).
3. As stated above (Question , Articles ,2), the New Law is called the law of faith, in so far as its pre-eminence is derived from that very grace which is given inwardly to believers, and for this reason is called the grace of faith. Nevertheless it consists secondarily in certain deeds, moral and sacramental: but the New Law does not consist chiefly in these latter things, as did the Old Law. As to those under the Old Testament who through faith were acceptable to God, in this respect they belonged to the New Testament: for they were not justified except through faith in Christ, Who is the Author of the New Testament. Hence of Moses the Apostle says (He 11,26) that he esteemed "the reproach of Christ greater riches than the treasure of the Egyptians."
Objection: 1. It would seem that the New Law does not fulfil the Old. Because to fulfil and to void are contrary. But the New Law voids or excludes the observances of the Old Law: for the Apostle says (Ga 5,2): "If you be circumcised, Christ shall profit you nothing." Therefore the New Law is not a fulfilment of the Old.
2. Further, one contrary is not the fulfilment of another. But Our Lord propounded in the New Law precepts that were contrary to precepts of the Old Law. For we read (Mt 5,27-32): You have heard that it was said to them of old: . . . "Whosoever shall put away his wife, let him give her a bill of divorce. But I say to you that whosoever shall put away his wife . . . maketh her to commit adultery." Furthermore, the same evidently applies to the prohibition against swearing, against retaliation, and against hating one's enemies. In like manner Our Lord seems to have done away with the precepts of the Old Law relating to the different kinds of foods (Mt 15,11): "Not that which goeth into the mouth defileth the man: but what cometh out of the mouth, this defileth a man." Therefore the New Law is not a fulfilment of the Old.
3. Further, whoever acts against a law does not fulfil the law. But Christ in certain cases acted against the Law. For He touched the leper (Mt 8,3), which was contrary to the Law. Likewise He seems to have frequently broken the sabbath; since the Jews used to say of Him (Jn 9,16): "This man is not of God, who keepeth not the sabbath." Therefore Christ did not fulfil the Law: and so the New Law given by Christ is not a fulfilment of the Old.
4. Further, the Old Law contained precepts, moral, ceremonial, and judicial, as stated above (Question , Article ). But Our Lord (Mt 5) fulfilled the Law in some respects, but without mentioning the judicial and ceremonial precepts. Therefore it seems that the New Law is not a complete fulfilment of the Old.
On the contrary Our Lord said (Mt 5,17): "I am not come to destroy, but to fulfil": and went on to say (Mt 5,18): "One jot or one tittle shall not pass of the Law till all be fulfilled."
I answer that As stated above (Article ), the New Law is compared to the Old as the perfect to the imperfect. Now everything perfect fulfils that which is lacking in the imperfect. And accordingly the New Law fulfils the Old by supplying that which was lacking in the Old Law.Now two things of every law is to make men righteous and virtuous, as was stated above (Question , Article ): and consequently the end of the Old Law was the justification of men. The Law, however, could not accomplish this: but foreshadowed it by certain ceremonial actions, and promised it in words. And in this respect, the New Law fulfils the Old by justifying men through the power of Christ's Passion. This is what the Apostle says (Rm 8,3-4): "What the Law could not do . . . God sending His own Son in the likeness of sinful flesh . . . hath condemned sin in the flesh, that the justification of the Law might be fulfilled in us." And in this respect, the New Law gives what the Old Law promised, according to 2Co 1,20: "Whatever are the promises of God, in Him," i.e. in Christ, "they are 'Yea'." [*The Douay version reads thus: "All the promises of God are in Him, 'It is'."] Again, in this respect, it also fulfils what the Old Law foreshadowed. Hence it is written (Col 2,17) concerning the ceremonial precepts that they were "a shadow of things to come, but the body is of Christ"; in other words, the reality is found in Christ. Wherefore the New Law is called the law of reality; whereas the Old Law is called the law of shadow or of figure.Now Christ fulfilled the precepts of the Old Law both in His works and in His doctrine. In His works, because He was willing to be circumcised and to fulfil the other legal observances, which were binding for the time being; according to Ga 4,4: "Made under the Law." In His doctrine He fulfilled the precepts of the Law in three ways. First, by explaining the true sense of the Law. This is clear in the case of murder and adultery, the prohibition of which the Scribes and Pharisees thought to refer only to the exterior act: wherefore Our Lord fulfilled the Law by showing that the prohibition extended also to the interior acts of sins. Secondly, Our Lord fulfilled the precepts of the Law by prescribing the safest way of complying with the statutes of the Old Law. Thus the Old Law forbade perjury: and this is more safely avoided, by abstaining altogether from swearing, save in cases of urgency. Thirdly, Our Lord fulfilled the precepts of the Law, by adding some counsels of perfection: this is clearly seen in Mt 19,21, where Our Lord said to the man who affirmed that he had kept all the precepts of the Old Law: "One thing is wanting to thee: If thou wilt be perfect, go, sell whatsoever thou hast," etc. [*St. Thomas combines Mt 19,21 with Mc 10,21].
Reply to Objection: 1. The New Law does not void observance of the Old Law except in the point of ceremonial precepts, as stated above (Question , Articles ,4). Now the latter were figurative of something to come. Wherefore from the very fact that the ceremonial precepts were fulfilled when those things were accomplished which they foreshadowed, it follows that they are no longer to be observed: for it they were to be observed, this would mean that something is still to be accomplished and is not yet fulfilled. Thus the promise of a future gift holds no longer when it has been fulfilled by the presentation of the gift. In this way the legal ceremonies are abolished by being fulfilled.
2. As Augustine says (Contra Faust. xix, 26), those precepts of Our Lord are not contrary to the precepts of the Old Law. For what Our Lord commanded about a man not putting away his wife, is not contrary to what the Law prescribed. "For the Law did not say: 'Let him that wills, put his wife away': the contrary of which would be not to put her away. On the contrary, the Law was unwilling that a man should put away his wife, since it prescribed a delay, so that excessive eagerness for divorce might cease through being weakened during the writing of the bill. Hence Our Lord, in order to impress the fact that a wife ought not easily to be put away, allowed no exception save in the case of fornication." The same applies to the prohibition about swearing, as stated above. The same is also clear with respect to the prohibition of retaliation. For the Law fixed a limit to revenge, by forbidding men to seek vengeance unreasonably: whereas Our Lord deprived them of vengeance more completely by commanding them to abstain from it altogether. With regard to the hatred of one's enemies, He dispelled the false interpretation of the Pharisees, by admonishing us to hate, not the person, but his sin. As to discriminating between various foods, which was a ceremonial matter, Our Lord did not forbid this to be observed: but He showed that no foods are naturally unclean, but only in token of something else, as stated above (Question , Article , ad 1).
3. It was forbidden by the Law to touch a leper; because by doing so, man incurred a certain uncleanness of irregularity, as also by touching the dead, as stated above (Question , Article , ad 4). But Our Lord, Who healed the leper, could not contract an uncleanness. By those things which He did on the sabbath, He did not break the sabbath in reality, as the Master Himself shows in the Gospel: both because He worked miracles by His Divine power, which is ever active among things; and because He worked miracles by His Divine power, which is ever active among things; and because His works were concerned with the salvation of man, while the Pharisees were concerned for the well-being of animals even on the sabbath; and again because on account of urgency He excused His disciples for gathering the ears of corn on the sabbath. But He did seem to break the sabbath according to the superstitious interpretation of the Pharisees, who thought that man ought to abstain from doing even works of kindness on the sabbath; which was contrary to the intention of the Law.
4. The reason why the ceremonial precepts of the Law are not mentioned in Mt 5 is because, as stated above (ad 1), their observance was abolished by their fulfilment. But of the judicial precepts He mentioned that of retaliation: so that what He said about it should refer to all the others. With regard to this precept, He taught that the intention of the Law was that retaliation should be sought out of love of justice, and not as a punishment out of revengeful spite, which He forbade, admonishing man to be ready to suffer yet greater insults; and this remains still in the New Law.
Objection: 1. It would seem that the New Law is not contained in the Old. Because the New Law consists chiefly in faith: wherefore it is called the "law of faith" (Rm 3,27). But many points of faith are set forth in the New Law, which are not contained in the Old. Therefore the New Law is not contained in the Old.
2. Further, a gloss says on Mt 5,19, "He that shall break one of these least commandments," that the lesser commandments are those of the Law, and the greater commandments, those contained in the Gospel. Now the greater cannot be contained in the lesser. Therefore the New Law is not contained in the Old.
3. Further, who holds the container holds the contents. If, therefore, the New Law is contained in the Old, it follows that whoever had the Old Law had the New: so that it was superfluous to give men a New Law when once they had the Old. Therefore the New Law is not contained in the Old.
On the contrary As expressed in Ez 1,16, there was "a wheel in the midst of a wheel," i.e. "the New Testament within the Old," according to Gregory's exposition.
I answer that One thing may be contained in another in two ways. First, actually; as a located thing is in a place. Secondly, virtually; as an effect in its cause, or as the complement in that which is incomplete; thus a genus contains its species, and a seed contains the whole tree, virtually. It is in this way that the New Law is contained in the Old: for it has been stated (Article ) that the New Law is compared to the Old as perfect to imperfect. Hence Chrysostom, expounding Mc 4,28, "The earth of itself bringeth forth fruit, first the blade, then the ear, afterwards the full corn in the ear," expresses himself as follows: "He brought forth first the blade, i.e. the Law of Nature; then the ear, i.e. the Law of Moses; lastly, the full corn, i.e. the Law of the Gospel." Hence then the New Law is in the Old as the corn in the ear.
Reply to Objection: 1. Whatsoever is set down in the New Testament explicitly and openly as a point of faith, is contained in the Old Testament as a matter of belief, but implicitly, under a figure. And accordingly, even as to those things which we are bound to believe, the New Law is contained in the Old.
2. The precepts of the New Law are said to be greater than those of the Old Law, in the point of their being set forth explicitly. But as to the substance itself of the precepts of the New Testament, they are all contained in the Old. Hence Augustine says (Contra Faust. xix, 23,28) that "nearly all Our Lord's admonitions or precepts, where He expressed Himself by saying: 'But I say unto you,' are to be found also in those ancient books. Yet, since they thought that murder was only the slaying of the human body, Our Lord declared to them that every wicked impulse to hurt our brother is to be looked on as a kind of murder." And it is in the point of declarations of this kind that the precepts of the New Law are said to be greater than those of the Old. Nothing, however, prevents the greater from being contained in the lesser virtually; just as a tree is contained in the seed.
3. What is set forth implicitly needs to be declared explicitly. Hence after the publishing of the Old Law, a New Law also had to be given.
Summa Th. I-II EN Qu.106 a.3