The great mediæval schoolman Peter Lombard, in Distinction 18.6 in his Sentences, may surprise papists with his understanding of the priest’s power to bind and loose. He says,
But he [i.e., God] did not grant this power [i.e., of remitting sin] to priests although he did grant them the power of binding and loosing, that is, of showing that men are bound or loosed – wherefore the Lord himself first restored the leper to health, and then sent him to the priests, by whose decision he was shown to be cleansed; so also after Lazarus was brought to life he gave him to the disciples to be loosed – because even if anyone is loosed by God, he is not on that account held to be loosed in the sight of the Church, except through the judgment of the priest. Therefore in loosing or retaining guilt, the priest of the Gospel so works and judges as the priest of the law in times past for those who were contaminated with leprosy, which signifies sin. Wherefore Jerome in his commentary on Matthew, where the Lord says to Peter: ‘I will give thee the keys of the kingdom of heaven, and whatsoever thou shalt bind on earth, shall be bound also in heaven and whatsoever thou shalt loose on earth, shall be loosed in heaven’: ‘Some persons who do not understand this passage’, he says, ‘assume something of the superciliousness of the Pharisees, so that they think they are to condemn the harmless and loose the harmful, when it is not the opinion of the priests, but the life of the doers, which is regarded by God. In Leviticus the lepers are commanded to show themselves to the priests, not that they make them lepers or clean, but that they distinguish who are clean or unclean; so also here.’ – It is here clearly shown that God does not always follow the judgment of the Church which judges sometimes dishonestly and ignorantly; but God always judges according to truth. And in remitting or retaining sins the priests of the Gospel have the power and office, which formerly the priests of the Law had under the Law in curing lepers. They therefore remit or retain sins, in so far as they judge and declare them to be remitted or retained by God. For the priests invoke ‘the name of the Lord on the sons of Israel, but he himself blesses,’ as we read in Numbers. This way of binding and loosing Jerome indicated above.
According to Lombard, then, the priest does have power from God to judge and speak for the Church on whether someone is bound or loosed; yet we must distinguish this judgement from that of God himself, who always judges according to truth and does not always agree with the Church.