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.
Objection 2. Further, the keys are given when orders are conferred. But knowledge is not always due to infusion, but sometimes is acquired, nor is it possessed by all those who are ordained, and is possessed by some who are not ordained. Therefore knowledge is not a key, so that there is but one key, viz. the power of judging.
Objection 3. Further, the power which the priest has over the mystic body of Christ flows from the power which he has over Christ's true body. Now the power of consecrating Christ's true body is but one. Therefore the power which regards Christ's mystic body is but one. But this is a key. Therefore, etc.
Objection 4. On the other hand, It seems that there are more than two keys. For just as knowledge and power are requisite for man to act, so is will. But the knowledge of discretion is reckoned as a key, and so is the power of judging. Therefore the will to absolve should be counted as a key.
Objection 5. Further, all three Divine Persons remit sins. Now the priest, through the keys, is the minister for the remission of sins. Therefore he should have three keys, so that he may be conformed to the Trinity.
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; nor can there be judgment, without the right to enforce it, since judgment is determined to one particular thing, which determination it derives, in speculative matters, from the first principles which cannot be gainsaid, and, in practical matters, from the power of command vested in the one who judges. And since the act of the key requires fitness in the person on whom it is exercised--because the ecclesiastical judge, by means of the key, "admits the worthy and excludes the unworthy," as may be seen from the definition given above (Article 2)--therefore the judge requires both judgment of discretion whereby he judges a man to be worthy, and also the very act of receiving (that man's confession); and for both these things a certain power or authority is necessary. Accordingly we may distinguish two keys, the first of which regards the judgment about the worthiness of the person to be absolved, while the other regards the absolution.
These two keys are distinct, not in the essence of authority, since both belong to the minister by virtue of his office, but in comparison with their respective acts, one of which presupposes the other.
Reply to Objection 1. One key is ordained immediately to the opening of one lock, but it is not unfitting that one key should be ordained to the act of another. Thus it is in the case in point. For it is the second key, which is the power of binding and loosing, that opens the lock of sin immediately, but the key of knowledge shows to whom that lock should be opened.
Reply to Objection 2. There are two opinions about the key of knowledge. For some say that knowledge considered as a habit, acquired or infused, is the key in this case, and that it is not the principal key, but is called a key through being subordinate to another key: so that it is not called a key when the other key is wanting, for instance, in an educated man who is not a priest. 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. This seems to have been the opinion of the Master (Sent. iv, D, 19).
But this does not seem to agree with the words of the Gospel, whereby the keys are promised to Peter (Matthew 16:19), so that not only one but two are given in orders. For which reason the other opinion holds that the key is not knowledge considered as a habit, but the authority to exercise the act of knowledge, which authority is sometimes without knowledge, while the knowledge is sometimes present without the authority. This may be seen even in secular courts, for a secular judge may have the authority to judge, without having the knowledge of the law, while another man, on the contrary, has knowledge of the law without having the authority to judge. And since the act of judging to which a man is bound through the authority which is vested in him, and not through his habit of knowledge, cannot be well performed without both of the above, the authority to judge, which is the key of knowledge, cannot be accepted without sin by one who lacks knowledge; whereas knowledge void of authority can be possessed without sin.
Reply to Objection 3. The power of consecrating is directed to only one act of another kind, wherefore it is not numbered among the keys, nor is it multiplied as the power of the keys, which is directed to different acts, although as to the essence of power and authority it is but one, as stated above.
Reply to Objection 4. Everyone is free to will, so that no one needs authority to will; wherefore will is not reckoned as a key.
Reply to Objection 5. All three Persons remit sins in the same way as one Person, wherefore there is no need for the priest, who is the minister of the Trinity, to have three keys: and all the more, since the will, which is appropriated to the Holy Ghost, requires no key, as stated above (ad 4).
Source: Summa Theologica (New Advent)