The Enchiridion by St. Augustine
Chapter 74.--God Does Not Pardon the Sins of Those Who Do Not from the Heart Forgive Others.
Now, he who asks forgiveness of the man against whom he has sinned, being moved by his sin to ask forgiveness, cannot be counted an enemy in such a sense that it should be as difficult to love him now as it was when he was engaged in active hostility. And the man who does not from his heart forgive him who repents of his sin, and asks forgiveness, need not suppose that his own sins are forgiven of God. For the Truth cannot lie. And what reader or hearer of the Gospel can have failed to notice, that the same person who said, |I am the Truth,| taught us also this form of prayer; and in order to impress this particular petition deeply upon our minds, said, |For if ye forgive men their trespasses, your heavenly Father will also forgive you; but if ye forgive not men their trespasses, neither will your Father forgive your trespasses|? The man whom the thunder of this warning does not awaken is not asleep, but dead; and yet so powerful is that voice, that it can awaken even the dead.