In his collected Works, Volume I, page 61, John Flavel, the Puritan pastor, helps us imagine the conversation between the Father and the Son in eternity past, planning our salvation. With the language updated a bit, it goes like this:
"Father: My Son, here is a company of poor miserable souls that have utterly undone themselves and now lie open to my justice. Justice demands satisfaction for them or will satisfy itself in the eternal ruin of them. What shall be done for these souls?
Son: O my Father, such is my love and pity for them that, rather than they shall perish eternally, I will be responsible for them as their Guarantee. Bring in all your bills, that I may see what they owe you. Lord, bring them all in, that there may be no after-reckonings with them. At my hand you shall require it. I will rather choose to suffer your wrath than they should suffer it. Upon me, my Father, upon me be all their debt.
Father: But my Son, if you undertake for them, you must reckon to pay the last cent. Expect no discounts. If I spare them, I will not spare you.
Son: Content, Father. Let it be so. Charge it all to me. I am able to pay it. And though it prove a kind of undoing to me, though it impoverish all my riches, empty all my treasures, yet I am content to undertake it."
On that Good Friday so long ago, the ancient agreement was fulfilled. He paid it all, down to the last cent. It is finished.