Objection 1. It would seem that the priesthood of Christ does not endure for ever. For as stated above (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 Isaiah 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.
Objection 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" (Hebrews 9:12). But the passion and death of Christ will not endure for ever, as stated Romans 6:9: "Christ rising again from the dead, dieth now no more." Therefore the priesthood of Christ will not endure for ever.
Objection 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 (Psalm 109:4): "Thou art a priest for ever."
I answer that, In the priestly office, we may consider two things: first, the offering of the sacrifice; secondly, the consummation of the sacrifice, consisting in this, that those for whom the sacrifice is offered, obtain the end of the sacrifice. Now the end of the sacrifice which Christ offered consisted not in temporal but in eternal good, which we obtain through His death, according to Hebrews 9:11: "Christ is [Vulgate: 'being come'] a high-priest of the good things to come"; for which reason the priesthood of Christ is said to be eternal. Now this consummation of Christ's sacrifice was foreshadowed in this, that the high-priest of the Old Law, once a year, entered into the Holy of Holies with the blood of a he-goat and a calf, as laid down, Leviticus 16:11, and yet he offered up the he-goat and calf not within the Holy of Holies, but without. In like manner Christ entered into the Holy of Holies--that is, into heaven--and prepared the way for us, that we might enter by the virtue of His blood, which He shed for us on earth.
Reply to Objection 1. The Saints who will be in heaven will not need any further expiation by the priesthood of Christ, but having expiated, they will need consummation through Christ Himself, on Whom their glory depends, as is written (Apocalypse 21:23): "The glory of God hath enlightened it"--that is, the city of the Saints--"and the Lamb is the lamp thereof."
Reply to Objection 2. Although Christ's passion and death are not to be repeated, yet the virtue of that Victim endures for ever, for, as it is written (Hebrews 10:14), "by one oblation He hath perfected for ever them that are sanctified."
Wherefore the reply to the third objection is clear.
As to the unity of this sacrifice, it was foreshadowed in the Law in that, once a year, the high-priest of the Law entered into the Holies, with a solemn oblation of blood, as set down, Leviticus 16:11. But the figure fell short of the reality in this, that the victim had not an everlasting virtue, for which reason those sacrifices were renewed every year.
Source: Summa Theologica (New Advent)