When you forgive, the debt doesn't just go away. You absorb the debt. That's why forgiveness is so hard.
Say you lend a person one thousand dollars, but suppose that when the
time comes for the loan to be repaid that person is unable to repay
you. If you forgive that person of his debt, then that person no longer
owes you anything. By forgiving, you forfeited the money—you absorbed
the debt.
How is it that God is able to forgive our sins against him? He
absorbed them by sending his son to suffer the penalty for our debts
(sins). Christ absorbed the penalty of death so that we don't have to.
When God forgave us, he said, "There's no need to repay me." (We
couldn't repay God even if we tried, but now we're freed from the burden
of trying.)
When you forgive someone, you absorb their debt. That's why forgiveness is so hard.
In Luke 7, Jesus tells this parable to Simon: "Two men owed money
to a certain moneylender. One owed him five hundred denarii, and the
other fifty. Neither of them had the money to pay him back, so he
canceled the debts of both. Now which of them will love him more?" Simon
replied, "I suppose the one who had the bigger debt canceled." "You
have judged correctly," Jesus said.
Forgiveness is a sign of love. To forgive someone of a massive debt
(in this case, five hundred denarii) is a sign of massive love. How is
forgiveness a sign of love? Because if you forgive much, then you must
absorb much debt (pain, suffering, loss, etc.). Our sin against God is
the biggest possible debt we could have; so for God to absorb that is a
sign of his endless, measureless love.
When He forgave us, God didn't just ignore our debt; he absorbed our debt.