Commentary Par XXIX 61-63

See Singleton's gloss (DDP Singleton.Par.XXIX.61-63): 'The angels who waited for the bestowal of the higher light, the light of glory, received that light (here termed "grazia illuminante").  Their merit (merto) was precisely that humility and their waiting upon the Lord to bestow that higher light.  With that bestowal they were forever confirmed in this highest grace, and accordingly they are now bound thereby to the good and to do the good.  They have fullness of vision and of will, and they cannot sin.  See Thomas Aquinas, Summa theol. I, q. 62, a. 8, resp.: "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."'