Objection 1. It would seem that the angels were created before the corporeal world. For Jerome says (In Ep. ad Tit. i, 2): "Six thousand years of our time have not yet elapsed; yet how shall we measure the time, how shall we count the ages, in which the Angels, Thrones, Dominations, and the other orders served God?" Damascene also says (De Fide Orth. ii): "Some say that the angels were begotten before all creation; as Gregory the Theologian declares, He first of all devised the angelic and heavenly powers, and the devising was the making thereof."
Objection 2. Further, the angelic nature stands midway between the Divine and the corporeal natures. But the Divine nature is from eternity; while corporeal nature is from time. Therefore the angelic nature was produced ere time was made, and after eternity.
Objection 3. Further, the angelic nature is more remote from the corporeal nature than one corporeal nature is from another. But one corporeal nature was made before another; hence the six days of the production of things are set forth in the opening of Genesis. Much more, therefore, was the angelic nature made before every corporeal nature.
On the contrary, It is said (Genesis 1:1): "In the beginning God created heaven and earth." Now, this would not be true if anything had been created previously. Consequently the angels were not created before corporeal nature.
I answer that, There is a twofold opinion on this point to be found in the writings of the Fathers. The more probable one holds that the angels were created at the same time as corporeal creatures. For the angels are part of the universe: they do not constitute a universe of themselves; but both they and corporeal natures unite in constituting one universe. This stands in evidence from the relationship of creature to creature; because the mutual relationship of creatures makes up the good of the universe. But no part is perfect if separate from the whole. Consequently it is improbable that God, Whose "works are perfect," as it is said Deuteronomy 32:4, should have created the angelic creature before other creatures. At the same time the contrary is not to be deemed erroneous; especially on account of the opinion of Gregory Nazianzen, "whose authority in Christian doctrine is of such weight that no one has ever raised objection to his teaching, as is also the case with the doctrine of Athanasius," as Jerome says.
Reply to Objection 1. Jerome is speaking according to the teaching of the Greek Fathers; all of whom hold the creation of the angels to have taken place previously to that of the corporeal world.
Reply to Objection 2. God is not a part of, but far above, the whole universe, possessing within Himself the entire perfection of the universe in a more eminent way. But an angel is a part of the universe. Hence the comparison does not hold.
Reply to Objection 3. All corporeal creatures are one in matter; while the angels do not agree with them in matter. Consequently the creation of the matter of the corporeal creature involves in a manner the creation of all things; but the creation of the angels does not involve creation of the universe.
If the contrary view be held, then in the text of Genesis 1, "In the beginning God created heaven and earth," the words, "In the beginning," must be interpreted, "In the Son," or "In the beginning of time": but not, "In the beginning, before which there was nothing," unless we say "Before which there was nothing of the nature of corporeal creatures."
Source: Summa Theologica (New Advent)