To get saved, we need to be forgiven of an impossible debt to God that only He can pay (Christ's sacrifice, being God, was perfect and infinite value); we must declare spiritual bankruptcy in God's court. We do not have a chance to meet God's demands and it's good that Jesus paid it in full by declaring: "It is finished [Tetelestai]." Which in interpretation, means and implies "Paid in Full." Once you have remitted a bill, you forget it! Same with God, once we are forgiven at salvation, God keeps no record of wrong and doesn't count our sins against us. (cf. 2 Cor. 5:19; Psalm 32:2). We no longer are debtors to God after our salvation because God no longer can count our sins against us or there would be double jeopardy and God is just and cannot allow His justice to be perverted.
We owe an outstanding debt to God and man to love and this the summation of our ethic: loving God and our neighbor. We even owe a moral debt or obligation to society in general! We forgive others then as Christ has forgiven us! We return the favor and be examples of forgiveness without any malice or vengeance for trespasses against us. Our forgiveness is not conditional because that would not be grace.
Just like Jesus said in the Lord's Prayer that we should forgive others their debts or sins as God forgives us. This is not legalism or adding to the grace of God: Jesus plus this or that. The statement is highly troubling and some think that if they haven't forgiven someone, they are lost. But we want to forgive out of gratitude and the operative word is "as" meaning we will do it. That is not the same as saying "if we forgive others." All our sins are forgiven upon salvation and we need only confess them to regain fellowship, not salvation. The woman forgiven of adultery loved Jesus so much that Jesus was prompted to say that he who is forgiven much, loves much!
God's grace is not only necessary for salvation, but sufficient. We forgive others out of love and a change of heart because God has transformed us from the inside out to be new creatures in Christ eager to do good and love others. Whenever we partake of the Lord's Supper, we are to examine ourselves to see if anyone holds something against us or we have sinned against a brother, even offending them or making them to fall.
God's mercy means that God doesn't treat us as we deserve and punish us, and His grace means He give us what we do not deserve as a blessing. We must never come to God seeking "justice," for if we got that, we would certainly be condemned. Some wish God were only "fair" to them and don't realize that would not be living by grace and we would forfeit the grace of God. If we deserved salvation, it would be justice, not mercy. The cost of unforgiveness is damnation! It does cost to be saved, but more not to be! Soli Deo Gloria!