Thomas Aquinas

“ I answer that, Whenever an act requires fitness on the part of the recipient, two things are necessary in the one who has to perform the act, viz. judgment of the fitness of the recipient, and accomplishment of the act. Therefore in the act of justice whereby a man is given what he deserves, there needs to be a judgment in order to discern whether he deserves to receive. Again, an authority or power is necessary for both these things, for we cannot give save what we have in our power ”

“ A person may be worthy to have something in two ways, either so as to have a right to possess it, and thus whoever is worthy has heaven already opened to him---or so that it is meet that he should receive it, and thus the power of the keys admits those who are worthy, but to whom heaven is not yet altogether opened. ”

“ The key whereby hell is opened and closed, is the power of bestowing grace, whereby hell is opened to man, so that he is taken out from sin which is the door of hell, and closed, so that by the help of grace man should no more fall into sin. ”

“ The priest's act does not bear immediately on the kingdom, but on the sacraments, by means of which man wins to the kingdom.
Art. 3 - Whether there are two keys or only one? Objection 1: It would seem that there are not two keys but only one. For one lock requires but one key. Now the lock for the removal of which the keys of the Church are required, is sin. Therefore the Church does not require two keys for one sin. ”

“ And although priests lack this key at times, through being without knowledge, acquired or infused, of loosing and binding, yet sometimes they make use of their natural endeavors, which they who hold this opinion call a little key, so that although knowledge be not bestowed together with orders, yet with the conferring of orders the knowledge becomes a key which it was not before. ”

“ Even as God hardens not by imparting malice, but by withholding grace, so a priest is said to exclude, not as though he placed an obstacle to entrance, but because he does not remove an obstacle which is there, since he cannot remove it unless God has already removed it. ”