What do you have that you did not receive? If then you received it, why do you boast as if you did not receive it? (1 Corinthians 4:7)
Picture salvation as a house that 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 all-satisfying glory.
God built it at great cost to himself and to his Son, and he gave it to you free and clear.
The “purchase” agreement is called a “new covenant.” The terms read: “This house shall become and remain yours if you will receive it 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 nor turn your heart away after other treasures, but find your contentment in the fellowship of God in this house.”
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 and paying for the house?
You would be treating the house no longer as 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 of putting on you.
If grace is to be free — which is the very meaning of grace — we cannot view it as something to be repaid.