7 Apply yourselves then earnestly to prayer, you sinners: confess your sins, pray that they may be blotted out, pray that they may be diminished, pray that as you increase, they may decrease: yet do not despair, and sinners though ye be, pray. For who has not sinned? Begin with the priests. To the priests it is said, “First offer sacrifices for your own sins, and so for the people.” The sacrifices convicted the priests that if any one should call himself righteous and without sin, it might be answered him, “I look not at what you say, but at what you offer; your own victim convicts you.
Wherefore do you offer for your own sins, if you have no sins? Do you in your sacrifice lie unto God?” But perhaps the priests of the ancient people were sinners; of the new people are not sinners. Of a truth, Brethren, for that God has so willed, I am His priest; I am a sinner; with you do I beat the breast, with you I ask for pardon, with you I hope that God will be merciful. But perhaps the Holy Apostles, those first and highest leaders of the flock, shepherds, members of The Shepherd, these perhaps had no sin.
Yes, indeed, even they had, they had indeed; they are not angry at this, for they confess it. I should not dare. First hear the Lord Himself saying to the Apostles, “In this manner pray ye.” As those other priests were convicted by the sacrifices, so these by prayer. And among the other things which He commanded them to pray for, He appointed this also, “Forgive us our debts, as we also forgive our debtors.” What do the Apostles say? Every day they pray for their debts to be forgiven them. They come in debtors, they go out absolved, and return debtors to prayer. This life is not without sin, that as often as prayer is made, so often should sins be forgiven.
Source: Sermons on Selected Lessons of the New Testament (New Advent)