Objection 1. It would seem that the Old Law should not have been given to the Jews alone. For the Old Law disposed men for the salvation which was to come through Christ, as stated above (2,3). But that salvation was to come not to the Jews alone but to all nations, according to Isaiah 49:6: "It is a small thing that thou shouldst be my servant to raise up the tribes of Jacob, and to convert the dregs of Israel. Behold I have given thee to be the light of the Gentiles, that thou mayest be My salvation, even to the farthest part of the earth." Therefore the Old Law should have been given to all nations, and not to one people only.
Objection 2. Further, according to Acts 10:34-35, "God is not a respecter of persons: but in every nation, he that feareth Him, and worketh justice, is acceptable to Him." Therefore the way of salvation should not have been opened to one people more than to another.
Objection 3. Further, the law was given through the angels, as stated above (Article 3). But God always vouchsafed the ministrations of the angels not to the Jews alone, but to all nations: for it is written (Sirach 17:14): "Over every nation He set a ruler." Also on all nations He bestows temporal goods, which are of less account with God than spiritual goods. Therefore He should have given the Law also to all peoples.
On the contrary, It is written (Romans 3:1-2): "What advantage then hath the Jew?... Much every way. First indeed, because the words of God were committed to them": and (Psalm 147:9): "He hath not done in like manner to every nation: and His judgments He hath not made manifest unto them."
I answer that, It might be assigned as a reason for the Law being given to the Jews rather than to other peoples, that the Jewish people alone remained faithful to the worship of one God, while the others turned away to idolatry; wherefore the latter were unworthy to receive the Law, lest a holy thing should be given to dogs.
But this reason does not seem fitting: because that people turned to idolatry, even after the Law had been made, which was more grievous, as is clear from Exodus 32 and from Amos 5:25-26: "Did you offer victims and sacrifices to Me in the desert for forty years, O house of Israel? But you carried a tabernacle for your Moloch, and the image of your idols, the star of your god, which you made to yourselves." Moreover it is stated expressly (Deuteronomy 9:6): "Know therefore that the Lord thy God giveth thee not this excellent land in possession for thy justices, for thou art a very stiff-necked people": but the real reason is given in the preceding verse: "That the Lord might accomplish His word, which He promised by oath to thy fathers Abraham, Isaac, and Jacob."
What this promise was is shown by the Apostle, who says (Galatians 3:16) that "to Abraham were the promises made and to his seed. He saith not, 'And to his seeds,' as of many: but as of one, 'And to thy seed,' which is Christ." And so God vouchsafed both the Law and other special boons to that people, on account of the promised made to their fathers that Christ should be born of them. For it was fitting that the people, of whom Christ was to be born, should be signalized by a special sanctification, according to the words of Leviticus 19:2: "Be ye holy, because I... am holy." Nor again was it on account of the merit of Abraham himself that this promise was made to him, viz. that Christ should be born of his seed: but of gratuitous election and vocation. Hence it is written (Isaiah 41:2): "Who hath raised up the just one form the east, hath called him to follow him?"
It is therefore evident that it was merely from gratuitous election that the patriarchs received the promise, and that the people sprung from them received the law; according to Deuteronomy 4:36-37: "Ye did [Vulgate: 'Thou didst'] hear His words out of the midst of the fire, because He loved thy fathers, and chose their seed after them." And if again it asked why He chose this people, and not another, that Christ might be born thereof; a fitting answer is given by Augustine (Tract. super Joan. xxvi): "Why He draweth one and draweth not another, seek not thou to judge, if thou wish not to err."
Reply to Objection 1. Although the salvation, which was to come through Christ, was prepared for all nations, yet it was necessary that Christ should be born of one people, which, for this reason, was privileged above other peoples; according to Romans 9:4: "To whom," namely the Jews, "belongeth the adoption as of children (of God)... and the testament, and the giving of the Law... whose are the fathers, and of whom is Christ according to the flesh."
Reply to Objection 2. Respect of persons takes place in those things which are given according to due; but it has no place in those things which are bestowed gratuitously. Because he who, out of generosity, gives of his own to one and not to another, is not a respecter of persons: but if he were a dispenser of goods held in common, and were not to distribute them according to personal merits, he would be a respecter of persons. Now God bestows the benefits of salvation on the human race gratuitously: wherefore He is not a respecter of persons, if He gives them to some rather than to others. Hence Augustine says (De Praedest. Sanct. viii): "All whom God teaches, he teaches out of pity; but whom He teaches not, out of justice He teaches not": for this is due to the condemnation of the human race for the sin of the first parent.
Reply to Objection 3. The benefits of grace are forfeited by man on account of sin: but not the benefits of nature. Among the latter are the ministries of the angels, which the very order of various natures demands, viz. that the lowest beings be governed through the intermediate beings: and also bodily aids, which God vouchsafes not only to men, but also to beasts, according to Psalm 35:7: "Men and beasts Thou wilt preserve, O Lord."
Source: Summa Theologica (New Advent)