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