Objection 1. It would seem that the angels were created in beatitude. For it is stated (De Eccl. Dogm. xxix) that "the angels who continue in the beatitude wherein they were created, do not of their nature possess the excellence they have." Therefore the angels were created in beatitude.
Objection 2. Further, the angelic nature is nobler than the corporeal creature. But the corporeal creature straightway from its creation was made perfect and complete; nor did its lack of form take precedence in time, but only in nature, as Augustine says (Gen. ad lit. i, 15). Therefore neither did God create the angelic nature imperfect and incomplete. But its formation and perfection are derived from its beatitude, whereby it enjoys God. Therefore it was created in beatitude.
Objection 3. Further, according to Augustine (Gen. ad lit. iv, 34; v, 5), the things which we read of as being made in the works of the six days, were made together at one time; and so all the six days must have existed instantly from the beginning of creation. But, according to his exposition, in those six days, "the morning" was the angelic knowledge, according to which they knew the Word and things in the Word. Therefore straightway from their creation they knew the Word, and things in the Word. But the bliss of the angels comes of seeing the Word. Consequently the angels were in beatitude straightway from the very beginning of their creation.
On the contrary, To be established or confirmed in good is of the nature of beatitude. But the angels were not confirmed in good as soon as they were created; the fall of some of them shows this. Therefore the angels were not in beatitude from their creation.
I answer that, By the name of beatitude is understood the ultimate perfection of rational or of intellectual nature; and hence it is that it is naturally desired, since everything naturally desires its ultimate perfection. Now there is a twofold ultimate perfection of rational or of intellectual nature. The first is one which it can procure of its own natural power; and this is in a measure called beatitude or happiness. Hence Aristotle (Ethic. x) says that man's ultimate happiness consists in his most perfect contemplation, whereby in this life he can behold the best intelligible object; and that is God. Above this happiness there is still another, which we look forward to in the future, whereby "we shall see God as He is." This is beyond the nature of every created intellect, as was shown above (Question 12, Article 4).
So, then, it remains to be said, that, as regards this first beatitude, which the angel could procure by his natural power, he was created already blessed. Because the angel does not acquire such beatitude by any progressive action, as man does, but, as was observed above (58, 3,4), is straightway in possession thereof, owing to his natural dignity. But the angels did not have from the beginning of their creation that ultimate beatitude which is beyond the power of nature; because such beatitude is no part of their nature, but its end; and consequently they ought not to have it immediately from the beginning.
Reply to Objection 1. Beatitude is there taken for that natural perfection which the angel had in the state of innocence.
Reply to Objection 2. The corporeal creature instantly in the beginning of its creation could not have the perfection to which it is brought by its operation; consequently, according to Augustine (Gen. ad. lit. v, 4,23; viii, 3), the growing of plants from the earth did not take place at once among the first works, in which only the germinating power of the plants was bestowed upon the earth. In the same way, the angelic creature in the beginning of its existence had the perfection of its nature; but it did not have the perfection to which it had to come by its operation.
Reply to Objection 3. The angel has a twofold knowledge of the Word; the one which is natural, and the other according to glory. He has a natural knowledge whereby he knows the Word through a similitude thereof shining in his nature; and he has a knowledge of glory whereby he knows the Word through His essence. By both kinds of knowledge the angel knows things in the Word; imperfectly by his natural knowledge, and perfectly by his knowledge of glory. Therefore the first knowledge of things in the Word was present to the angel from the outset of his creation; while the second was not, but only when the angels became blessed by turning to the good. And this is properly termed their morning knowledge.
Source: Summa Theologica (New Advent)