Nicene and Post-Nicene Fathers, Vol. V:
A Treatise Concerning Man’s Perfection...: Section 6

(6.) The Sixth Breviate.

VI. “Again,” he says, “we have to inquire whether man is commanded to be without sin; for either he is not able, and then he is not commanded; or else because he is commanded, he is able. For why should that be commanded which cannot at all be done?” The answer is, that man is most wisely commanded to walk with right steps, on purpose that, when he has discovered his own inability to do even this, he may seek the remedy which is provided for the inward man to cure the lameness of sin, even the grace of God, through our Lord Jesus Christ.