“And forgive us our debts, as we also have forgiven our debtors.” Matthew 6:12

Forgive us our debts: In Matthew, sins are called debts, because they expose us to condemnation at the tribunal of God, and make us debtors; nay more, they alienate us entirely from God, so that there is no hope of obtaining peace and favor except by pardon. For, though the righteousness of God shines, to some extent, in the saints, yet, so long as they are surrounded by the flesh, they lie under the burden of sins. None will be found so pure as not to need the mercy of God, and if we wish to partake of it, we must feel our wretchedness. 

As we forgive our debtors: This condition is added, that no one may presume to approach God and ask forgiveness, who is not pure and free from all resentment. And yet the forgiveness, which we ask that God would give us, does not depend on the forgiveness which we grant to others: but the design of Christ was, to exhort us, in this manner, to forgive the offenses which have been committed against us, and at the same time, to give, as it were, the impression of his seal, to ratify the confidence in our own forgiveness [and to] remind us of the feelings which we ought to cherish towards brethren, when we desire to be reconciled to God. And certainly, if the Spirit of God reigns in our hearts, every description of ill-will and revenge ought to be banished. The Spirit is the witness of our adoption, (Romans 8:16) and therefore this is put down simply as a mark, to distinguish the children of God from strangers. The name debtors is here given, not to those who owe us money, or any other service, but to those who are indebted to us on account of offenses which they have committed.

John Calvin, The Lord’s Prayer Commentary