I have looked fruitlessly for this quotation a number of times, and I finally just stumbled across it:
It is not biblical to say that the only virtues God can require of me are the ones that I am good enough to perform. If I am so bad that I can’t delight in what is good, that is no reason God can’t command me to love the good. If I am so corrupt that I can’t enjoy what is infinitely beautiful, that does not make me less guilty for disobeying the command to delight in God (Ps. 37:4). It makes me more guilty.
—John Piper, When I Don’t Desire God: How to Fight for Joy (Wheaton, IL: Crossway, 2004), 47.