Objection 1. It would seem that the angels have no cause of their existence. For the first chapter of Genesis treats of things created by God. But there is no mention of angels. Therefore the angels were not created by God.
Objection 2. Further, the Philosopher says (Metaph. viii, text. 16) that if any substance be a form without matter, "straightway it has being and unity of itself, and has no cause of its being and unity." But the angels are immaterial forms, as was shown above (Question 50, Article 2). Therefore they have no cause of their being.
Objection 3. Further, whatever is produced by any agent, from the very fact of its being produced, receives form from it. But since the angels are forms, they do not derive their form from any agent. Therefore the angels have no active cause.
On the contrary, It is said (Psalm 148:2): "Praise ye Him, all His angels"; and further on, verse 5: "For He spoke and they were made."
I answer that, It must be affirmed that angels and everything existing, except God, were made by God. God alone is His own existence; while in everything else the essence differs from the existence, as was shown above (Question 3, Article 4). From this it is clear that God alone exists of His own essence: while all other things have their existence by participation. Now whatever exists by participation is caused by what exists essentially; as everything ignited is caused by fire. Consequently the angels, of necessity, were made by God.
Reply to Objection 1. Augustine says (De Civ. Dei xi, 50) that the angels were not passed over in that account of the first creation of things, but are designated by the name "heavens" or of "light." And they were either passed over, or else designated by the names of corporeal things, because Moses was addressing an uncultured people, as yet incapable of understanding an incorporeal nature; and if it had been divulged that there were creatures existing beyond corporeal nature, it would have proved to them an occasion of idolatry, to which they were inclined, and from which Moses especially meant to safeguard them.
Reply to Objection 2. Substances that are subsisting forms have no 'formal' cause of their existence and unity, nor such active cause as produces its effect by changing the matter from a state of potentiality to actuality; but they have a cause productive of their entire substance.
From this the solution of the third difficulty is manifest.
Source: Summa Theologica (New Advent)