Objection 1. It would seem that the angels know mysteries of grace. For, the mystery of the Incarnation is the most excellent of all mysteries. But the angels knew of it from the beginning; for Augustine says (Gen. ad lit. v, 19): "This mystery was hidden in God through the ages, yet so that it was known to the princes and powers in heavenly places." And the Apostle says (1 Timothy 3:16): "That great mystery of godliness appeared unto angels." Therefore the angels know the mysteries of grace.
Objection 2. Further, the reasons of all mysteries of grace are contained in the Divine wisdom. But the angels behold God's wisdom, which is His essence. Therefore they know the mysteries of grace.
Objection 3. Further, the prophets are enlightened by the angels, as is clear from Dionysius (Coel. Hier. iv). But the prophets knew mysteries of grace; for it is said (Amos 3:7): "For the Lord God doth nothing without revealing His secret to His servants the prophets." Therefore angels know the mysteries of grace.
On the contrary, No one learns what he knows already. Yet even the highest angels seek out and learn mysteries of grace. For it is stated (Coel. Hier. vii) that "Sacred Scripture describes some heavenly essences as questioning Jesus, and learning from Him the knowledge of His Divine work for us; and Jesus as teaching them directly": as is evident in Isaiah 63:1, where, on the angels asking, "Who is he who cometh up from Edom?" Jesus answered, "It is I, Who speak justice." Therefore the angels do not know mysteries of grace.
I answer that, There is a twofold knowledge in the angel. The first is his natural knowledge, according to which he knows things both by his essence, and by innate species. By such knowledge the angels cannot know mysteries of grace. For these mysteries depend upon the pure will of God: and if an angel cannot learn the thoughts of another angel, which depend upon the will of such angel, much less can he ascertain what depends entirely upon God's will. The Apostle reasons in this fashion (1 Corinthians 2:11): "No one knoweth the things of a man [Vulgate: 'What man knoweth the things of a man, but...?'], but the spirit of a man that is in him." So, "the things also that are of God no man knoweth but the Spirit of God."
There is another knowledge of the angels, which renders them happy; it is the knowledge whereby they see the Word, and things in the Word. By such vision they know mysteries of grace, but not all mysteries: nor do they all know them equally; but just as God wills them to learn by revelation; as the Apostle says (1 Corinthians 2:10): "But to us God hath revealed them through His Spirit"; yet so that the higher angels beholding the Divine wisdom more clearly, learn more and deeper mysteries in the vision of God, which mysteries they communicate to the lower angels by enlightening them. Some of these mysteries they knew from the very beginning of their creation; others they are taught afterwards, as befits their ministrations.
Reply to Objection 1. One can speak in two ways of the mystery of the Incarnation.
First of all, in general; and in this way it was revealed to all from the commencement of their beatitude. The reason of this is, that this is a kind of general principle to which all their duties are ordered. For "all are [Vulgate: 'Are they not all.'] ministering spirits, sent to minister for them who shall receive the inheritance of salvation (Hebrews 1:14)"; and this is brought by the mystery of the Incarnation. Hence it was necessary for all of them to be instructed in this mystery from the very beginning.
We can speak of the mystery of the Incarnation in another way, as to its special conditions. Thus not all the angels were instructed on all points from the beginning; even the higher angels learned these afterwards, as appears from the passage of Dionysius already quoted.
Reply to Objection 2. Although the angels in bliss behold the Divine wisdom, yet they do not comprehend it. So it is not necessary for them to know everything hidden in it.
Reply to Objection 3. Whatever the prophets knew by revelation of the mysteries of grace, was revealed in a more excellent way to the angels. And although God revealed in general to the prophets what He was one day to do regarding the salvation of the human race, still the apostles knew some particulars of the same, which the prophets did not know. Thus we read (Ephesians 3:4-5): "As you reading, may understand my knowledge in the mystery of Christ, which in other generations was not known to the sons of men, as it is now revealed to His holy apostles." Among the prophets also, the later ones knew what the former did not know; according to Psalm 118:100: "I have had understanding above ancients," and Gregory says: "The knowledge of Divine things increased as time went on" (Hom. xvi in Ezech.).
Source: Summa Theologica (New Advent)