Objection 1. It would seem that the highest among the angels who sinned was not the highest of all. For it is stated (Ezekiel 28:14): "Thou wast a cherub stretched out, and protecting, and I set thee in the holy mountain of God." Now the order of the Cherubim is under the order of the Seraphim, as Dionysius says (Coel. Hier. vi, vii). Therefore, the highest angel among those who sinned was not the highest of all.
Objection 2. Further, God made intellectual nature in order that it might attain to beatitude. If therefore the highest of the angels sinned, it follows that the Divine ordinance was frustrated in the noblest creature which is unfitting.
Objection 3. Further, the more a subject is inclined towards anything, so much the less can it fall away from it. But the higher an angel is, so much the more is he inclined towards God. Therefore so much the less can he turn away from God by sinning. And so it seems that the angel who sinned was not the highest of all, but one of the lower angels.
On the contrary, Gregory (Hom. xxxiv in Ev.) says that the chief angel who sinned, "being set over all the hosts of angels, surpassed them in brightness, and was by comparison the most illustrious among them."
I answer that, Two things have to be considered in sin, namely, the proneness to sin, and the motive for sinning. If, then, in the angels we consider the proneness to sin, it seems that the higher angels were less likely to sin than the lower. On this account Damascene says (De Fide Orth. ii), that the highest of those who sinned was set over the terrestrial order. This opinion seems to agree with the view of the Platonists, which Augustine quotes (De Civ. Dei vii, 6,7; x, 9,10,11). For they said that all the gods were good; whereas some of the demons were good, and some bad; naming as 'gods' the intellectual substances which are above the lunar sphere, and calling by the name of "demons" the intellectual substances which are beneath it, yet higher than men in the order of nature. Nor is this opinion to be rejected as contrary to faith; because the whole corporeal creation is governed by God through the angels, as Augustine says (De Trin. iii, 4,5). Consequently there is nothing to prevent us from saying that the lower angels were divinely set aside for presiding over the lower bodies, the higher over the higher bodies; and the highest to stand before God. And in this sense Damascene says (De Fide Orth. ii) that they who fell were of the lower grade of angels; yet in that order some of them remained good.
But if the motive for sinning be considered, we find that it existed in the higher angels more than in the lower. For, as has been said (2), the demons' sin was pride; and the motive of pride is excellence, which was greater in the higher spirits. Hence Gregory says that he who sinned was the very highest of all. This seems to be the more probable view: because the angels' sin did not come of any proneness, but of free choice alone. Consequently that argument seems to have the more weight which is drawn from the motive in sinning. Yet this must not be prejudicial to the other view; because there might be some motive for sinning in him also who was the chief of the lower angels.
Reply to Objection 1. Cherubim is interpreted "fulness of knowledge," while "Seraphim" means "those who are on fire," or "who set on fire." Consequently Cherubim is derived from knowledge; which is compatible with mortal sin; but Seraphim is derived from the heat of charity, which is incompatible with mortal sin. Therefore the first angel who sinned is called, not a Seraph, but a Cherub.
Reply to Objection 2. The Divine intention is not frustrated either in those who sin, or in those who are saved; for God knows beforehand the end of both; and He procures glory from both, saving these of His goodness, and punishing those of His justice. But the intellectual creature, when it sins, falls away from its due end. Nor is this unfitting in any exalted creature; because the intellectual creature was so made by God, that it lies within its own will to act for its end.
Reply to Objection 3. However great was the inclination towards good in the highest angel, there was no necessity imposed upon him: consequently it was in his power not to follow it.
Source: Summa Theologica (New Advent)