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. TP, Question [48], Article [6]; FS, Question [112], Article [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.