Objection 1. It would seem that holy men, even those who are not priests, have the use of the keys. For loosing and binding, which are the effects of the keys, derive their efficacy from the merit of Christ's Passion. Now those are most conformed to Christ's Passion, who follow Christ, suffering by patience and other virtues. Therefore it seems that even if they have not the priestly order, they can bind and loose.
Objection 2. Further, it is written (Hebrews 7:7): "Without all contradiction, that which is less is blessed by the greater [Vulgate: 'better']." Now "in spiritual matters," according to Augustine (De Trin. vi, 8), "to be better is to be greater." Therefore those who are better, i.e. who have more charity, can bless others by absolving them. Hence the same conclusion follows.
On the contrary, "Action belongs to that which has the power," as the Philosopher says (De Somno et Vigil. i). But the key which is a spiritual power belongs to priests alone. Therefore priests alone are competent to have the use of the keys.
I answer that, There is this difference between a principal and an instrumental agent, that the latter does not produce, in the effect, its own likeness, but the likeness of the principal agent, whereas the principal agent produces its own likeness. Consequently a thing becomes a principal agent through having a form, which it can reproduce in another, whereas an instrumental agent is not constituted thus, but through being applied by the principal agent in order to produce a certain effect. Since therefore in the act of the keys the principal agent by authority is Christ as God, and by merit is Christ as man,* it follows that on account of the very fulness of Divine goodness in Him, and of the perfection of His grace, He is competent to exercise the act of the keys. [For St. Thomas' later teaching on this point, cf. III, 48, 6; I-II, 112, 1, ad 1. But another man is not competent to exercise this act as principal agent, since neither can he give another man grace whereby sins are remitted, nor can he merit sufficiently, so that he is nothing more than an instrumental agent. Consequently the recipient of the effect of the keys, is likened, not to the one who uses the keys, but to Christ. Therefore, no matter how much grace a man may have, he cannot produce the effect of the keys, unless he be appointed to that purpose by receiving orders.
Reply to Objection 1. Just as between instrument and effect there is need or likeness, not of a similar form, but of aptitude in the instrument for the effect, so is it as regards the instrument and the principal agent. The former is the likeness between holy men and the suffering Christ, nor does it bestow on them the use of the keys.
Reply to Objection 2. Although a mere man cannot merit grace for another man condignly, yet the merit of one man can co-operate in the salvation of another. Hence there is a twofold blessing. One proceeds from a mere man, as meriting by his own act: this blessing can be conferred by any holy person in whom Christ dwells by His grace, in so far as he excels in goodness the person whom he blesses. The other blessing is when a man blesses, as applying a blessing instrumentally through the merit of Christ, and this requires excellence of order and not of virtue.
Source: Summa Theologica (New Advent)