"Picture salvation as a house you live in. It provides you with protection. It is stocked with food and drink that will last forever. It never decays or crumbles. Its windows open onto vistas of glory. God built at great cost to himself and to his Son, and he gave it to you. The “purchase” agreement is called a “new covenant.” The terms read: “this house shall become and remain yours if you will receive as a gift and take delight in the Father and the Son as they inhabit the house with you. You shall not profane the house of God by sheltering other gods, or turn your heart away after other treasures.” Would it not be foolish to say yes to this agreement, and then hire a lawyer to draw up an amortization schedule with monthly payments in the hopes of somehow balancing accounts. You would be treating the house no longer is a gift, but a purchase. God would no longer be the free benefactor. And you would be enslaved to a new set of demands that he never dreamed putting on you. If grace is to be free–which is the very meaning of grace–we can have you as something to be repaid."
-John Piper; Future Grace