16 But God wanted not power to make man such as that he should not be able to sin: but He chose rather to make him such, as that it should lie in his power to sin, if he would; not to sin, if he would not; forbidding the one, enjoining the other; that it might be to him first a good desert not to sin, and after a just reward not to be able to sin. For such also at the last will He makes His Saints, as to be without all power to sin. Such forsooth even now has He His angels, whom in Him we so love, as to have no fear for any of them, lest by sinning he become a devil. And this we presume not of any just man in this mortal life. But we trust that all will be such in that immortal life. For Almighty God Who works good even of our evils, what good will He give, when He shall have set us free from all evils? Much may be said more fully and more subtilely on the good use of evil; but this is not what we have undertaken in our present discourse, and we must avoid in it excess of length.
Source: On Continence (New Advent)