On page 98 of Miroslav Volf's Free of Charge: Giving and Forgiving, he says:
But even if all of us are sinners from head to toe, none of us is a sinner through and through, with nothing good remaining in us. As sinners, we are still God's good creatures. To illustrate the relationship between being a good creature and being a sinner, Reformation theologians used the analogy of water and ink. Water is the good creation, ink is sin, and the sinner is a glass of water with a few drops of ink. All the water in the glass is tainted, but it's still mostly water, not ink. Analogously, all our good deeds are marred by sin, but they are still mostly good deeds, not crimes masquerading as merits. Now apply this to gifts. We give gifts. None of them are pure. Yet with all their impurities, many of them are still genuine gifts, not just hidden ways of loving ourselves.
There is no footnote. Who is Volf most likely referring to? What is the original context(s)?