That is the mystery: We must obey the command to rejoice in the Lord, and we cannot, because of our willful and culpable corruption. Therefore obedience, when it happens, is a gift. The heretic Pelagius in the fourth century rejected this truth and was shocked and angered when he saw St. Augustine prayed in his Confessions. Augustine prayed, “Give me the grace [O Lord] to do as you command, and command me to do what you will!…O holy God…when your commands are obeyed, it is from you that we receive the power to obey them.”
– Piper, John; When I Don’t Desire God: How to Fight for Joy; copyright 2004; Crossway Books; Wheaton, Il.; p. 53