“Lastly, it is to be observed that the condition of being forgiven as we forgive our debtors, is not added because by forgiving others we deserve forgiveness, as if the cause of forgiveness were expressed; but by the use of this expression the Lord has been pleased partly to solace the weakness of our faith, using it as a sign to assure us that our sins are as certainly forgiven as we are certainly conscious of having forgiven others, when our mind is completely purged from all envy, hatred, and malice; and partly using as a badge by which he excludes from the number of his children all who, prone to revenge and reluctant to forgive, obstinately keep up their enmity, cherishing against others that indignation which they deprecate from themselves; so that they should not venture to invoke him as a Father. In the Gospel of Luke, we have this distinctly stated in the words of Christ.”
John Calvin, Institutes, III.XX.45