17 Whence does this appear? From the very Prayer itself. “For if,” says He, “ye forgive men their debts, your heavenly Father will forgive your debts.” And as much as the difference is between “a hundred pence” and “ten thousand talents,” so great is it between the debts on the one side, and those on the other!
What punishment then must he not deserve, who when he would receive ten thousand talents, in the room of a hundred pence, yet will not even so remit this small sum, but offers up the Prayer against himself. For when you say, “Forgive us, as we forgive,” and afterwards dost not forgive, you are supplicating of God nothing else than that He would entirely deprive you of all excuse or indulgence. “But I do not presume to say,” replies some one, “Forgive me as I forgive” but only, “Forgive me.” But what matters this? For if you say it not yourself, yet God so does; as you forgive, He forgives. And this He has made quite evident from what follows; for there it is said, “If you forgive not men, neither does your heavenly Father forgive you.” Think not, therefore, that it is a pious caution, not to repeat the whole sentence; nor offer up the Prayer by halves, but as He bade you so pray thou, in order that the very obligation of that expression, putting you daily in fear, may compel you to the exercise of forgiveness towards your neighbours.
Source: Homilies on the Statues (New Advent)