“But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.”
Matthew 6:15 — Greek Interlinear
Church Fathers on Matthew 6:15
By the word, 'Amen.' He shews that without doubt the Lord will bestow all things that are rightly asked, and by those that do not fail in observing the annexed condition, 'For if ye forgive men their sins, your heavenly Father will also forgive you your sins.'
Here we should not overlook that of all the petitions enjoined by the Lord, He judged that most worthy of further enforcement, which relates to forgiveness of sins, in which He would have us merciful; which is the only means of escaping misery.
He does not say that God will first forgive us, and that we should after forgive our debtors. For God knows how treacherous the heart of man is, and that though they should have received forgiveness themselves, yet they do not forgive their debtors; therefore He instructs us first to forgive, and we shall be forgiven after.