6. They affirm that they are showing great reverence for God, to Whom alone they reserve the power of forgiving sins. But in truth none do Him greater injury than they who choose to prune His commandments and reject the office entrusted to them. For inasmuch as the Lord Jesus Himself said in the Gospel: “Receive ye the Holy Spirit: whosesoever sins ye forgive they are forgiven unto them, and whosesoever sins ye retain, they are retained,” who is it that honours Him most, he who obeys His bidding or he who rejects it?
7. The Church holds fast its obedience on either side, by both retaining and remitting sin; heresy is on the one side cruel, and on the other disobedient; wishes to bind what it will not loosen, and will not loosen what it has bound, whereby it condemns itself by its own sentence. For the Lord willed that the power of binding and of loosing should be alike, and sanctioned each by a similar condition. So he who has not the power to loose has not the power to bind. For as, according to the Lord’s word, he who has the power to bind has also the power to loose, their teaching destroys itself, inasmuch as they who deny that they have the power of loosing ought also to deny that of binding. For how can the one be allowed and the other disallowed? It is plain and evident that either each is allowed or each is disallowed in the case of those to whom each has been given. Each is allowed to the Church, neither to heresy, for this power has been entrusted to priests alone. Rightly, therefore, does the Church claim it, which has true priests; heresy, which has not the priests of God, cannot claim it. And by not claiming this power heresy pronounces its own sentence, that not possessing priests it cannot claim priestly power.
St. Ambrose of Milan
Concerning Repentance 1.2.6-7