Q: Whether the Effect of Christ's Priesthood Is the Expiation of Sins?

A: On the contrary,
The Apostle says (Heb. 9:14): "The blood of Christ, Who by the Holy Ghost offered Himself unspotted unto God, shall cleanse our conscience from dead works, to serve the living God." But dead works denote sins. Therefore the priesthood of Christ has the power to cleanse from sins.

I answer that,
Two things are required for the perfect cleansing from sins, corresponding to the two things comprised in sin--namely, the stain of sin and the debt of punishment. The stain of sin is, indeed, blotted out by grace, by which the sinner's heart is turned to God: whereas the debt of punishment is entirely removed by the satisfaction that man offers to God. Now the priesthood of Christ produces both these effects. For by its virtue grace is given to us, by which our hearts are turned to God, according to Rom. 3:24, 25: "Being justified freely by His grace, through the redemption that is in Christ Jesus, Whom God hath proposed to be a propitiation, through faith in His blood." Moreover, He satisfied for us fully, inasmuch as "He hath borne our infirmities and carried our sorrows" (Isa. 53:4). Wherefore it is clear that the priesthood of Christ has full power to expiate sins.

Reply Obj. 1: Although Christ was a priest, not as God, but as man, yet one and the same was both priest and God. Wherefore in the Council of Ephesus [*Part III, ch. i, anath. 10] we read: "If anyone say that the very Word of God did not become our High-Priest and Apostle, when He became flesh and a man like us, but altogether another one, the man born of a woman, let him be anathema." Hence in so far as His human nature operated by virtue of the Divine, that sacrifice was most efficacious for the blotting out of sins. For this reason Augustine says (De Trin. iv, 14): "So that, since four things are to be observed in every sacrifice--to whom it is offered, by whom it is offered, what is offered, for whom it is offered; the same one true Mediator reconciling us to God by the sacrifice of peace, was one with Him to Whom it was offered, united in Himself those for whom He offered it, at the same time offered it Himself, and was Himself that which He offered."

Reply Obj. 2: Sins are commemorated in the New Law, not on account of the inefficacy of the priesthood of Christ, as though sins were not sufficiently expiated by Him: but in regard to those who either are not willing to be participators in His sacrifice, such as unbelievers, for whose sins we pray that they be converted; or who, after taking part in this sacrifice, fall away from it by whatsoever kind of sin. The Sacrifice which is offered every day in the Church is not distinct from that which Christ Himself offered, but is a commemoration thereof. Wherefore Augustine says (De Civ. De. x, 20): "Christ Himself both is the priest who offers it and the victim: the sacred token of which He wished to be the daily Sacrifice of the Church."

Reply Obj. 3: As Origen says (Sup. Joan. i, 29), though various animals were offered up under the Old Law, yet the daily sacrifice, which was offered up morning and evening, was a lamb, as appears from Num. 38:3, 4. By which it was signified that the offering up of the true lamb, i.e. Christ, was the culminating sacrifice of all. Hence (John 1:29) it is said: "Behold the Lamb of God, behold Him Who taketh away the sins [Vulg.: 'sin'] of the world."