Objection 1. It would seem that the key is not the power of binding and loosing, whereby "the ecclesiastical judge has to admit the worthy to the kingdom and exclude the unworthy" therefrom, as stated in the text (Sent. iv, D, 16). For the spiritual power conferred in a sacrament is the same as the character. But the key and the character do not seem to be the same, since by the character man is referred to God, whereas by the key he is referred to his subjects. Therefore the key is not a power.
Objection 2. Further, an ecclesiastical judge is only one who has jurisdiction, which is not given at the same time as orders. But the keys are given in the conferring of orders. Therefore there should have been no mention of the ecclesiastical judge in the definition of the keys.
Objection 3. Further, when a man has something of himself, he needs not to be reduced to act by some active power. Now a man is admitted to the kingdom from the very fact that he is worthy. Therefore it does not concern the power of the keys to admit the worthy to the kingdom.
Objection 4. Further, sinners are unworthy of the kingdom. But the Church prays for sinners, that they may go to heaven. Therefore she does not exclude the unworthy, but admits them, so far as she is concerned.
Objection 5. Further, in every ordered series of agents, the last end belongs to the principal and not to the instrumental agent. But the principal agent in view of man's salvation is God. Therefore admission to the kingdom, which is the last end, belongs to Him, and not to those who have the keys, who are as instrumental or ministerial agents.
I answer that, According to the Philosopher (De Anima ii, text. 33), "powers are defined from their acts." Wherefore, since the key is a kind of power, it should be defined from its act or use, and reference to the act should include its object from which it takes its species, and the mode of acting whereby the power is shown to be well-ordered. Now the act of the spiritual power is to open heaven, not absolutely, since it is already open, as stated above (1, ad 1), but for this or that man; and this cannot be done in an orderly manner without due consideration of the worthiness of the one to be admitted to heaven. Hence the aforesaid definition of the key gives the genus, viz. "power," the subject of the power, viz. the "ecclesiastical judge," and the act, viz. "of excluding or admitting," corresponding to the two acts of a material key which are to open and shut; the object of which act is referred to in the words "from the kingdom," and the mode, in the words, "worthy" and "unworthy," because account is taken of the worthiness or unworthiness of those on whom the act is exercised.
Reply to Objection 1. The same power is directed to two things, of which one is the cause of the other, as heat, in fire, is directed to make a thing hot and to melt it. And since every grace and remission in a mystical body comes to it from its head, it seems that it is essentially the same power whereby a priest can consecrate, and whereby he can loose and bind, if he has jurisdiction, and that there is only a logical difference, according as it is referred to different effects, even as fire in one respect is said to have the power of heating, and in another, the power of melting. And because the character of the priestly order is nothing else than the power of exercising that act to which the priestly order is chiefly ordained (if we maintain that it is the same as a spiritual power), therefore the character, the power of consecrating, and the power of the keys are one and the same essentially, but differ logically.
Reply to Objection 2. All spiritual power is conferred by some kind of consecration. Therefore the key is given together with the order: yet the use of the key requires due matter, i.e. a people subject through jurisdiction, so that until he has jurisdiction, the priest has the keys, but he cannot exercise the act of the keys. And since the key is defined from its act, its definition contains a reference to jurisdiction.
Reply to Objection 3. 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.
Reply to Objection 4. 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. [St. Thomas here follows the opinion of Peter Lombard, and replies in the negative. Later in life he altered his opinion. Cf. III, 62, 1; III, 64, 1; III, 86, 6.] Hence God is prayed that He may absolve, so that there may be room for the priest's absolution.
Reply to Objection 5. The priest's act does not bear immediately on the kingdom, but on the sacraments, by means of which man wins to the kingdom.
Source: Summa Theologica (New Advent)