Objection 1. It would seem that all men were bound to observe the Old Law. Because whoever is subject to the king, must needs be subject to his law. But the Old Law was given by God, Who is "King of all the earth" (Psalm 46:8). Therefore all the inhabitants of the earth were bound to observe the Law.
Objection 2. Further, the Jews could not be saved without observing the Old Law: for it is written (Deuteronomy 27:26): "Cursed be he that abideth not in the words of this law, and fulfilleth them not in work." If therefore other men could be saved without the observance of the Old Law, the Jews would be in a worse plight than other men.
Objection 3. Further, the Gentiles were admitted to the Jewish ritual and to the observances of the Law: for it is written (Exodus 12:48): "If any stranger be willing to dwell among you, and to keep the Phase of the Lord, all his males shall first be circumcised, and then shall he celebrate it according to the manner; and he shall be as he that is born in the land." But it would have been useless to admit strangers to the legal observances according to Divine ordinance, if they could have been saved without the observance of the Law. Therefore none could be saved without observing the Law.
On the contrary, Dionysius says (Coel. Hier. ix) that many of the Gentiles were brought back to God by the angels. But it is clear that the Gentiles did not observe the Law. Therefore some could be saved without observing the Law.
I answer that, The Old Law showed forth the precepts of the natural law, and added certain precepts of its own. Accordingly, as to those precepts of the natural law contained in the Old Law, all were bound to observe the Old Law; not because they belonged to the Old Law, but because they belonged to the natural law. But as to those precepts which were added by the Old Law, they were not binding on save the Jewish people alone.
The reason of this is because the Old Law, as stated above (Article 4), was given to the Jewish people, that it might receive a prerogative of holiness, in reverence for Christ Who was to be born of that people. Now whatever laws are enacted for the special sanctification of certain ones, are binding on them alone: thus clerics who are set aside for the service of God are bound to certain obligations to which the laity are not bound; likewise religious are bound by their profession to certain works of perfection, to which people living in the world are not bound. In like manner this people was bound to certain special observances, to which other peoples were not bound. Wherefore it is written (Deuteronomy 18:13): "Thou shalt be perfect and without spot before the Lord thy God": and for this reason they used a kind of form of profession, as appears from Deuteronomy 26:3: "I profess this day before the Lord thy God," etc.
Reply to Objection 1. Whoever are subject to a king, are bound to observe his law which he makes for all in general. But if he orders certain things to be observed by the servants of his household, others are not bound thereto.
Reply to Objection 2. The more a man is united to God, the better his state becomes: wherefore the more the Jewish people were bound to the worship of God, the greater their excellence over other peoples. Hence it is written (Deuteronomy 4:8): "What other nation is there so renowned that hath ceremonies and just judgments, and all the law?" In like manner, from this point of view, the state of clerics is better than that of the laity, and the state of religious than that of folk living in the world.
Reply to Objection 3. The Gentiles obtained salvation more perfectly and more securely under the observances of the Law than under the mere natural law: and for this reason they were admitted to them. So too the laity are now admitted to the ranks of the clergy, and secular persons to those of the religious, although they can be saved without this.
Source: Summa Theologica (New Advent)