From the Shorter Catechism

Q. 105. What do we pray for in the fifth petition?

A. In the fifth petition, which is, “And forgive us our debts, as we forgive our debtors,” we pray, that God, for Christ’s sake, would freely pardon all our sins; which we are the rather encouraged to ask, because by his grace we are enabled from the heart to forgive others. Matt. 6:12; Ps. 51:1-2, 7, 9; Dan. 9:17-19; Luke 11:4; Matt. 18:35.