Objection 1. It seems that the Old Law was not given through the angels, but immediately by God. For an angel means a "messenger"; so that the word "angel" denotes ministry, not lordship, according to Psalm 102:20-21: "Bless the Lord, all ye His Angels... you ministers of His." But the Old Law is related to have been given by the Lord: for it is written (Exodus 20:1): "And the Lord spoke... these words," and further on: "I am the Lord Thy God." Moreover the same expression is often repeated in Exodus, and the later books of the Law. Therefore the Law was given by God immediately.
Objection 2. Further, according to John 1:17, "the Law was given by Moses." But Moses received it from God immediately: for it is written (Exodus 33:11): "The Lord spoke to Moses face to face, as a man is wont to speak to his friend." Therefore the Old Law was given by God immediately.
Objection 3. Further, it belongs to the sovereign alone to make a law, as stated above (Question 90, Article 3). But God alone is Sovereign as regards the salvation of souls: while the angels are the "ministering spirits," as stated in Hebrews 1:14. Therefore it was not meet for the Law to be given through the angels, since it is ordained to the salvation of souls.
On the contrary, The Apostle said (Galatians 3:19) that the Law was "given [Vulgate: 'ordained'] by angels in the hand of a Mediator." And Stephen said (Acts 7:53): "(Who) have received the Law by the disposition of angels."
I answer that, The Law was given by God through the angels. And besides the general reason given by Dionysius (Coel. Hier. iv), viz. that "the gifts of God should be brought to men by means of the angels," there is a special reason why the Old Law should have been given through them. For it has been stated (1,2) that the Old Law was imperfect, and yet disposed man to that perfect salvation of the human race, which was to come through Christ. Now it is to be observed that wherever there is an order of powers or arts, he that holds the highest place, himself exercises the principal and perfect acts; while those things which dispose to the ultimate perfection are effected by him through his subordinates: thus the ship-builder himself rivets the planks together, but prepares the material by means of the workmen who assist him under his direction. Consequently it was fitting that the perfect law of the New Testament should be given by the incarnate God immediately; but that the Old Law should be given to men by the ministers of God, i.e. by the angels. It is thus that the Apostle at the beginning of his epistle to the Hebrews (1:2) proves the excellence of the New Law over the Old; because in the New Testament "God... hath spoken to us by His Son," whereas in the Old Testament "the word was spoken by angels" (Hebrews 2:2).
Reply to Objection 1. As Gregory says at the beginning of his Morals (Praef. chap. i), "the angel who is described to have appeared to Moses, is sometimes mentioned as an angel, sometimes as the Lord: an angel, in truth, in respect of that which was subservient to the external delivery; and the Lord, because He was the Director within, Who supported the effectual power of speaking." Hence also it is that the angel spoke as personating the Lord.
Reply to Objection 2. As Augustine says (Gen. ad lit. xii, 27), it is stated in Exodus that "the Lord spoke to Moses face to face"; and shortly afterwards we read, "Show me Thy glory. Therefore He perceived what he saw and he desired what he saw not." Hence he did not see the very Essence of God; and consequently he was not taught by Him immediately. Accordingly when Scripture states that "He spoke to him face to face," this is to be understood as expressing the opinion of the people, who thought that Moses was speaking with God mouth to mouth, when God spoke and appeared to him, by means of a subordinate creature, i.e. an angel and a cloud. Again we may say that this vision "face to face" means some kind of sublime and familiar contemplation, inferior to the vision of the Divine Essence.
Reply to Objection 3. It is for the sovereign alone to make a law by his own authority; but sometimes after making a law, he promulgates it through others. Thus God made the Law by His own authority, but He promulgated it through the angels.
Source: Summa Theologica (New Advent)