I answer that, As stated above (A. 1), a priest is set between God and man. Now he needs someone between himself and God, who of himself cannot approach to God; and such a one is subject to the priesthood by sharing in the effect thereof. But this cannot be said of Christ; for the Apostle says (Heb. 7:25): "Coming of Himself to God, always living to make intercession for us [Vulg.: 'He is able to save for ever them that come to God by Him; always living,' etc.]." And therefore it is not fitting for Christ to be the recipient of the effect of His priesthood, but rather to communicate it to others. For the influence of the first agent in every genus is such that it receives nothing in that genus: thus the sun gives but does not receive light; fire gives but does not receive heat. Now Christ is the fountain-head of the entire priesthood: for the priest of the Old Law was a figure of Him; while the priest of the New Law works in His person, according to 2 Cor. 2:10: "For what I have pardoned, if I have pardoned anything, for your sakes have I done it in the person of Christ." Therefore it is not fitting that Christ should receive the effect of His priesthood.
Reply Obj. 1: Although prayer is befitting to priests, it is not their proper office, for it is befitting to everyone to pray both for himself and for others, according to James 5:16: "Pray for one another that you may be saved." And so we may say that the prayer by which Christ prayed for Himself was not an action of His priesthood. But this answer seems to be precluded by the Apostle, who, after saying (Heb. 5:6), "Thou art a priest for ever according to the order of Melchisedech," adds, "Who in the days of His flesh offering up payers," etc., as quoted above (Obj. 1): so that it seems that the prayer which Christ offered pertained to His priesthood. We must therefore say that other priests partake in the effect of their priesthood, not as priests, but as sinners, as we shall state farther on (ad 3). But Christ had, simply speaking, no sin; though He had the "likeness of sin in the flesh [Vulg.: 'sinful flesh']," as is written Rom. 8:3. And, consequently, we must not say simply that He partook of the effect of His priesthood but with this qualification— in regard to the passibility of the flesh. Wherefore he adds pointedly, "that was able to save Him from death."
Reply Obj. 2: Two things may be considered in the offering of a sacrifice by any priest—namely, the sacrifice itself which is offered, and the devotion of the offerer. Now the proper effect of priesthood is that which results from the sacrifice itself. But Christ obtained a result from His passion, not as by virtue of the sacrifice, which is offered by way of satisfaction, but by the very devotion with which out of charity He humbly endured the passion.
Reply Obj. 3: A figure cannot equal the reality, wherefore the figural priest of the Old Law could not attain to such perfection as not to need a sacrifice of satisfaction. But Christ did not stand in need of this. Consequently, there is no comparison between the two; and this is what the Apostle says (Heb. 7:28): "The Law maketh men priests, who have infirmity; but the word of the oath, which was since the Law, the Son Who is perfected for evermore." _______________________
FIFTH ARTICLE [III, Q. 22, Art. 5]
Whether the Priesthood of Christ Endures for Ever?
Objection 1: It would seem that the priesthood of Christ does not endure for ever. For as stated above (A. 4, ad 1, 3) those alone need the effect of the priesthood who have the weakness of sin, which can be expiated by the priest's sacrifice. But this will not be for ever. For in the Saints there will be no weakness, according to Isa. 60:21: "Thy people shall be all just": while no expiation will be possible for the weakness of sin, since "there is no redemption in hell" (Office of the Dead, Resp. vii). Therefore the priesthood of Christ endures not for ever.
Obj. 2: Further, the priesthood of Christ was made manifest most of all in His passion and death, when "by His own blood He entered into the Holies" (Heb. 9:12). But the passion and death of Christ will not endure for ever, as stated Rom. 6:9: "Christ rising again from the dead, dieth now no more." Therefore the priesthood of Christ will not endure for ever.
Obj. 3: Further, Christ is a priest, not as God, but as man. But at one time Christ was not man, namely during the three days He lay dead. Therefore the priesthood of Christ endures not for ever.
On the contrary, It is written (Ps. 109:4): "Thou art a priest for ever."