4 This would I say even if you had no sin which you might desire to be forgiven you. But as it is, whosoever you are, you are a man; though you be righteous, you are a man; whether you are a layman, or monk, or clerk, or Bishop, or Apostle, you are a man. Hear the Apostle's voice, “If we shall say that we have no sin, we deceive ourselves.” He, that famous John and an Evangelist, he whom the Lord Christ loved beyond all the rest, who lay on His breast, he says, “If we shall say.”
He did not say, “If you shall say that you have no sin,” but “if we shall say that we have no sin, we deceive ourselves, and the truth is not in us.” He joined himself in the guilt, that he might be joined in the pardon also. “If we shall say.” Consider who it is that says, “If we shall say that we have no sin, we deceive ourselves, and the truth is not in us. But if we shall confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all iniquity.” How does He cleanse?
By forgiving, not as though He found nothing to punish, but as finding something to forgive. So then, Brethren, if we have sins, let us forgive them that ask us. Let us not retain enmities in our heart against another. For the retaining of enmities more than anything corrupts this heart of ours.
Source: Sermons on Selected Lessons of the New Testament (New Advent)