Objection 1. It would seem that there is but one Divine law. Because, where there is one king in one kingdom there is but one law. Now the whole of mankind is compared to God as to one king, according to Psalm 46:8: "God is the King of all the earth." Therefore there is but one Divine law.
Objection 2. Further, every law is directed to the end which the lawgiver intends for those for whom he makes the law. But God intends one and the same thing for all men; since according to 1 Timothy 2:4: "He will have all men to be saved, and to come to the knowledge of the truth." Therefore there is but one Divine law.
Objection 3. Further, the Divine law seems to be more akin to the eternal law, which is one, than the natural law, according as the revelation of grace is of a higher order than natural knowledge. Therefore much more is the Divine law but one.
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 is twofold, as stated in the same passage, viz. the levitical priesthood, and the priesthood of Christ. Therefore the Divine law is twofold, namely the Old Law and the New Law.
I answer that, As stated in the I, 30, 3, distinction is the cause of number. Now things may be distinguished in two ways. First, as those things that are altogether specifically different, e.g. a horse and an ox. Secondly, as perfect and imperfect in the same species, e.g. a boy and a man: and in this way the Divine law is divided into Old and New. Hence the Apostle (Galatians 3:24-25) compares the state of man under the Old Law to that of a child "under a pedagogue"; but the state under the New Law, to that of a full grown man, who is "no longer under a pedagogue."
Now the perfection and imperfection of these two laws is to be taken in connection with the three conditions pertaining to law, as stated above. For, in the first place, it belongs to law to be directed to the common good as to its end, as stated above (Question 90, Article 2). This good may be twofold. It may be a sensible and earthly good; and to this, man was directly ordained by the Old Law: wherefore, at the very outset of the law, the people were invited to the earthly kingdom of the Chananaeans (Exodus 3:8-17). Again it may be an intelligible and heavenly good: and to this, man is ordained by the New Law. Wherefore, at the very beginning of His preaching, Christ invited men to the kingdom of heaven, saying (Matthew 4:17): "Do penance, for the kingdom of heaven is at hand." Hence Augustine says (Contra Faust. iv) that "promises of temporal goods are contained in the Old Testament, for which reason it is called old; but the promise of eternal life belongs to the New Testament."
Secondly, it belongs to the law to direct human acts according to the order of righteousness (4): wherein also the New Law surpasses the Old Law, since it directs our internal acts, according to Matthew 5:20: "Unless your justice abound more than that of the Scribes and Pharisees, you shall not enter into the kingdom of heaven." Hence the saying that "the Old Law restrains the hand, but the New Law controls the mind" (Sentent. iii, D, xl).
Thirdly, it belongs to the law to induce men to observe its commandments. This the Old Law did by the fear of punishment: but the New Law, by love, which is poured into our hearts by the grace of Christ, bestowed in the New Law, but foreshadowed in the Old. Hence Augustine says (Contra Adimant. Manich. discip. xvii) that "there is little difference [The 'little difference' refers to the Latin words 'timor' and 'amor'--'fear' and 'love.'] between the Law and the Gospel--fear and love."
Reply to Objection 1. As the father of a family issues different commands to the children and to the adults, so also the one King, God, in His one kingdom, gave one law to men, while they were yet imperfect, and another more perfect law, when, by the preceding law, they had been led to a greater capacity for Divine things.
Reply to Objection 2. The salvation of man could not be achieved otherwise than through Christ, according to Acts 4:12: "There is no other name... given to men, whereby we must be saved." Consequently the law that brings all to salvation could not be given until after the coming of Christ. But before His coming it was necessary to give to the people, of whom Christ was to be born, a law containing certain rudiments of righteousness unto salvation, in order to prepare them to receive Him.
Reply to Objection 3. The natural law directs man by way of certain general precepts, common to both the perfect and the imperfect: wherefore it is one and the same for all. But the Divine law directs man also in certain particular matters, to which the perfect and imperfect do not stand in the same relation. Hence the necessity for the Divine law to be twofold, as already explained.
Source: Summa Theologica (New Advent)