From John Piper’s When I Don’t Desire God –
“We are commanded to do what we cannot do. And we must do it or perish. Our inability does not remove our guilt. It deepens it. We are so bad that we cannot love God. We cannot delight in God above all things. We cannot treasure Christ above money. Our entrenchedbadness does not make it wrong for God to command us to be good. We ought to delight in God above all things. Therefore it is right for God to command us to delight in God above all things. And if we ever do delight n God, it will be because we have obeyed this command.
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 the way 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.”
We may paraphrase like this: “Let him who rejoices in the Lord, rejoice in the joy that God supplies, so that in everything including our joy God may be glorified through Jesus Christ.” Therefore, believing that joy in God is a gift from God is essential in our calling to live for the glory of God. It shapes all our other strategies. It makes them humble. It makes them into acts of faith. In everything we do in our quest for joy we are praying and trusting the grace of God for a gift. May this
truth liberate the despairing soul and humble the proud.”