Objection 1. It would seem that a beatified angel can sin. For, as was said above (Article 7), beatitude does not do away with nature. But it is of the very notion of created nature, that it can fail. Therefore a beatified angel can sin.
Objection 2. Further, the rational powers are referred to opposites, as the Philosopher observes (Metaph. iv, text. 3). But the will of the angel in beatitude does not cease to be rational. Therefore it is inclined towards good and evil.
Objection 3. Further, it belongs to the liberty of free-will for man to be able to choose good or evil. But the freedom of will is not lessened in the beatified angels. Therefore they can sin.
On the contrary, Augustine says (Gen. ad lit. xi) that "there is in the holy angels that nature which cannot sin." Therefore the holy angels cannot sin.
I answer that, The beatified angels cannot sin. The reason for this is, because their beatitude consists in seeing God through His essence. Now, God's essence is the very essence of goodness. Consequently the angel beholding God is disposed towards God in the same way as anyone else not seeing God is to the common form of goodness. Now it is impossible for any man either to will or to do anything except aiming at what is good; or for him to wish to turn away from good precisely as such. Therefore the beatified angel can neither will nor act, except as aiming towards God. Now whoever wills or acts in this manner cannot sin. Consequently the beatified angel cannot sin.
Reply to Objection 1. Created good, considered in itself, can fail. But from its perfect union with the uncreated good, such as is the union of beatitude, it is rendered unable to sin, for the reason already alleged.
Reply to Objection 2. The rational powers are referred to opposites in the things to which they are not inclined naturally; but as to the things whereunto they have a natural tendency, they are not referred to opposites. For the intellect cannot but assent to naturally known principles; in the same way, the will cannot help clinging to good, formally as good; because the will is naturally ordained to good as to its proper object. Consequently the will of the angels is referred to opposites, as to doing many things, or not doing them. But they have no tendency to opposites with regard to God Himself, Whom they see to be the very nature of goodness; but in all things their aim is towards God, which ever alternative they choose, that is not sinful.
Reply to Objection 3. Free-will in its choice of means to an end is disposed just as the intellect is to conclusions. Now it is evident that it belongs to the power of the intellect to be able to proceed to different conclusions, according to given principles; but for it to proceed to some conclusion by passing out of the order of the principles, comes of its own defect. Hence it belongs to the perfection of its liberty for the free-will to be able to choose between opposite things, keeping the order of the end in view; but it comes of the defect of liberty for it to choose anything by turning away from the order of the end; and this is to sin. Hence there is greater liberty of will in the angels, who cannot sin, than there is in ourselves, who can sin.
Source: Summa Theologica (New Advent)