Gratitude

This immediately excludes the debtor's ethic. Any attempt to express a gratitude by paying God back would contradict the nature of His gift as free and gracious. Any attempt to turn from being a beneficiary of God in order to become God's benefactors would remove the stumbling block of the cross where my debt was so fully paid that I am forever humbled to the status of a receiver, not a giver. "Whoever serves is to do so as one who is serving by the strength which God supplies" (1 Pet. 4:11 NASB)
Instead, the way our joy expresses the value of free grace is by admitting we don't deserve it, and by banking our hope on it and doing everything we do as a recipient of more and more grace. "God is able to make all grace abound to you, [so that] ...you may have an abundance for every good deed" (2 Cor. 9:8 NASB). Good deeds do not pay back grace; they borrow more grace.

John Piper, Brothers, We Are Not Professionals, pg. 38