Augustine of Hippo, On the Spirit and the Letter Chapter 2: The Examples Apposite

Here, perhaps, you will say to me in answer, that the things which I have instanced as not having been realized, although capable of realization, are divine works; whereas a man’s being without sin falls in the range of a man’s own work,—that being indeed his very noblest work which effects a full and perfect righteousness complete in every part; and therefore that it is incredible that no man has ever existed, or is existing, or will exist in this life, who has achieved such a work, if the achievement is possible for a human being. But then you ought to reflect that, although this great work, no doubt, belongs to human agency to accomplish, yet it is also a divine gift, and therefore, not doubt that it is a divine work; “for it is God who worketh in you both to will and to do of His good pleasure.”