Augustine of Hippo — on Phlm 1:14 (GRACE AND FREE WILL 2.4)

“And addressing himself to Philemon, Paul says: "that your kindness may not be as it were of necessity, but voluntary." … Now wherever there is the express statement not to do this or that, and whenever the performance of the will is required to do or refrain from some action, in keeping with God's commandments, that is sufficient proof of the free choice of the will. Let no one, therefore, blame God in his heart whenever he sins, but let him impute the sin to himself.”