In the explanation of Luther’s Small Catechism, one of the questions is: “What does it show when we forgive others? “ The answer given is: “It shows that we truly believe that God has forgiven us.” This comes from the section on the Lord’s Prayer, specifically about the line, “forgive us our debts,as we also have forgiven our debtors.” Or we might says, “forgive us our trespasses, as we forgive those who trespass against us.”
The Small Catechism also explains that we sin everyday and deserve nothing but God’s punishment, but in the Lord’s Prayer we ask for God’s gracious forgiveness because of Jesus Christ. And when we pray the Lord’s Prayer we are not worthy of the things we ask and pray for, so we ask for God’s forgiveness to pray to Him with a good conscience, and God wants us then to forgive and do good to those who sin against us because He forgives us.
Also we can listen to these words from the Large Catechism, which explain how we are sure that everything is forgiven and pardoned in the way that that we also forgive are neighbor:
“If therefore, you do not forgive, then do not think that God forgives you. But if you forgive, you have this comfort and assurance, that you are forgiven in heaven. This is not because of your forgiving. For God forgives freely and without condition, out of pure grace, because He has so promised, as the Gospel teaches. But God says this in order that He may establish forgiveness as our confirmation and assurance, as a sign alongside the promise, which agrees with this prayer in Luke 6:37, “Forgive, and you will be forgiven.” [598-99]