It is the covenant of redemption.
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 to and pity for them that rather than they shall perish eternally, I will be responsible for them as their Guarantee. Bring 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 will require it. I would rather choose to suffer your wrath than they suffer it. Upon me, my Father, upon me be all their debt.
Father: But my Son, if you undertake for them, you must pay the last penny. Expect no discounts. If I spare them, I will not spare you.
Son: I am willing, Father. Let it be so. Charge it all to me. I am able to pay their debt. And though it will undo me, though it will impoverish all my riches and empty all my accounts, yet I am content to undertake it.
-John Flavel on the covenant of redemption, quoting from a number of Scripture references:
John 4:34; 5:30, 36, 43; 6:37-40; 10:18; 12:49; 14:31; 15:10; 17:1-2, 4-6, 9-11, 24; 19:30
Ephesians 1:3-14; 2 Timothy 1:9