9 They answer, “knowing not,” as the Apostle says, “either what they speak, or whereof they affirm:” they answer and say, “If men do not forgive sins, then that is false which Christ says, 'Whatsoever you shall loose on earth, shall be loosed in heaven also.'” But you do not know why this is said, and in what sense this is said. The Lord was about to give to men the Holy Spirit, and He wished it to be understood that sins are forgiven to His faithful by His Holy Spirit, and not by men's deserts.
For what are you, O man, but an invalid who hast need of healing. Would you make yourself my physician? Together with me, seek the Physician. For that the Lord might show this more plainly, that sins are forgiven by the Holy Spirit, which He has given to His faithful ones, and not by men's deserts, after He had risen from the dead, He says in a certain place, “Receive the Holy Ghost;” and when He had said, “Receive the Holy Ghost,” He subjoined immediately, “Whosesoever sins ye remit, they are remitted unto them;” that is, the Spirit remits them, not ye.
Now the Spirit is God. God therefore remits, not ye. But what are you in regard to the Spirit? “Do you not know that you are the temple of God, and the Spirit of God dwells in you?” And again, “Do you not know that your bodies are the temples of the Holy Ghost which is in you, which you have of God?” So then God dwells in His holy temple, that is in His holy faithful ones, in His Church; by them does He remit sins; because they are living temples.
Source: Sermons on Selected Lessons of the New Testament (New Advent)