Our Lord comments on this: `So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses.' That is exactly the same teaching. But does it teach that I am forgiven only because I have forgiven? No, the teaching is, and we have to take this teaching seriously, that if I do not forgive, I am not forgiven. I explain it like this: the man who has seen himself as a guilty, vile sinner before God knows his only hope of heaven is that God has forgiven him freely. The man who truly sees and knows and believes that, is one who cannot refuse to forgive another. So the man who does not forgive another does not know forgiveness himself. If my heart has been broken in the presence of God I cannot refuse to forgive; and, therefore, I say to any man who is imagining fondly that his sins are to be forgiven by Christ, though he does not forgive anybody else, Beware, my friend, lest you wake up in eternity and find Him saying to you, `Depart from me; I never knew you.' You are misinterpreting the doctrine, the glorious doctrine of the grace of God. The man who is truly forgiven and knows it, is a man who forgives. That is the meaning of the Sermon on the Mount at this point.
D. Martyn Lloyd-Jones. Studies in the Sermon on the Mount. General Introduction.