Objection 1. It would seem that the angel was produced by God from eternity. For God is the cause of the angel by His being: for He does not act through something besides His essence. But His being is eternal. Therefore He produced the angels from eternity.
Objection 2. Further, everything which exists at one period and not at another, is subject to time. But the angel is above time, as is laid down in the book De Causis. Therefore the angel is not at one time existing and at another non-existing, but exists always.
Objection 3. Further, Augustine (De Trin. xiii) proves the soul's incorruptibility by the fact that the mind is capable of truth. But as truth is incorruptible, so is it eternal. Therefore the intellectual nature of the soul and of the angel is not only incorruptible, but likewise eternal.
On the contrary, It is said (Proverbs 8:22), in the person of begotten Wisdom: "The Lord possessed me in the beginning of His ways, before He made anything from the beginning." But, as was shown above (Article 1), the angels were made by God. Therefore at one time the angels were not.
I answer that, God alone, Father, Son and Holy Ghost, is from eternity. Catholic Faith holds this without doubt; and everything to the contrary must be rejected as heretical. For God so produced creatures that He made them "from nothing"; that is, after they had not been.
Reply to Objection 1. God's being is His will. So the fact that God produced the angels and other creatures by His being does not exclude that He made them also by His will. But, as was shown above (19, 3; 46, 1). God's will does not act by necessity in producing creatures. Therefore He produced such as He willed, and when He willed.
Reply to Objection 2. An angel is above that time which is the measure of the movement of the heavens; because he is above every movement of a corporeal nature. Nevertheless he is not above time which is the measure of the succession of his existence after his non-existence, and which is also the measure of the succession which is in his operations. Hence Augustine says (Gen. ad lit. viii, 20,21) that "God moves the spiritual creature according to time."
Reply to Objection 3. Angels and intelligent souls are incorruptible by the very fact of their having a nature whereby they are capable of truth. But they did not possess this nature from eternity; it was bestowed upon them when God Himself willed it. Consequently it does not follow that the angels existed from eternity.
Source: Summa Theologica (New Advent)