Objection 1. It would seem that the angel's intellect is sometimes in potentiality and sometimes in act. For movement is the act of what is in potentiality, as stated in Phys. iii, 6. But the angels' minds are moved by understanding, as Dionysius says (Div. Nom. iv). Therefore the angelic minds are sometimes in potentiality.
Objection 2. Further, since desire is of a thing not possessed but possible to have, whoever desires to know anything is in potentiality thereto. But it is said (1 Peter 1:12): "On Whom the angels desire to look." Therefore the angel's intellect is sometimes in potentiality.
Objection 3. Further, in the book De Causis it is stated that "an intelligence understands according to the mode of its substance." But the angel's intelligence has some admixture of potentiality. Therefore it sometimes understands potentially.
On the contrary, Augustine says (Gen. ad lit. ii): "Since the angels were created, in the eternity of the Word, they enjoy holy and devout contemplation." Now a contemplating intellect is not in potentiality, but in act. Therefore the intellect of an angel is not in potentiality.
I answer that, As the Philosopher states (De Anima iii, text. 8; Phys. viii, 32), the intellect is in potentiality in two ways; first, "as before learning or discovering," that is, before it has the habit of knowledge; secondly, as "when it possesses the habit of knowledge, but does not actually consider." In the first way an angel's intellect is never in potentiality with regard to the things to which his natural knowledge extends. For, as the higher, namely, the heavenly, bodies have no potentiality to existence, which is not fully actuated, in the same way the heavenly intellects, the angels, have no intelligible potentiality which is not fully completed by connatural intelligible species. But with regard to things divinely revealed to them, there is nothing to hinder them from being in potentiality: because even the heavenly bodies are at times in potentiality to being enlightened by the sun.
In the second way an angel's intellect can be in potentiality with regard to things learnt by natural knowledge; for he is not always actually considering everything that he knows by natural knowledge. But as to the knowledge of the Word, and of the things he beholds in the Word, he is never in this way in potentiality; because he is always actually beholding the Word, and the things he sees in the Word. For the bliss of the angels consists in such vision; and beatitude does not consist in habit, but in act, as the Philosopher says (Ethic. i, 8).
Reply to Objection 1. Movement is taken there not as the act of something imperfect, that is, of something existing in potentiality, but as the act of something perfect, that is, of one actually existing. In this way understanding and feeling are termed movements, as stated in De Anima iii, text. 28.
Reply to Objection 2. Such desire on the part of the angels does not exclude the object desired, but weariness thereof. Or they are said to desire the vision of God with regard to fresh revelations, which they receive from God to fit them for the tasks which they have to perform.
Reply to Objection 3. In the angel's substance there is no potentiality divested of act. In the same way, the angel's intellect is never so in potentiality as to be without act.
Source: Summa Theologica (New Advent)