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 Hebrews 11:6. But the faith of olden times and of nowadays is the same, as the gloss says on Matthew 21:9. Therefore the law is the same also.
Objection 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" (Leviticus 19:18), and: "Thou shalt love the Lord thy God" (Deuteronomy 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 Mark 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 Hebrews 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.
Objection 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" (Romans 3:27). But the Old Law was also a law of faith, according to Hebrews 11:39: "All were [Vulgate: '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 (Matthew 5:44): "Do good to them that hate you"; and (Luke 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 (Hebrews 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 (90, 2; 91, 4), 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 Romans 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 (Galatians 3:24), whereas the New Law is the law of perfection, since it is the law of charity, of which the Apostle says (Colossians 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 62, Article 2) 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.
Reply to Objection 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" (Romans 5:5).
Reply to Objection 3. As stated above (106, A1,2), 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 (Hebrews 11:26) that he esteemed "the reproach of Christ greater riches than the treasure of the Egyptians."
Source: Summa Theologica (New Advent)