I answer that, There is a twofold efficient agency—namely, the principal and the instrumental. Now the principal efficient cause of man's salvation is God. But since Christ's humanity is the "instrument of the Godhead," as stated above (Q. 43, A. 2), therefore all Christ's actions and sufferings operate instrumentally in virtue of His Godhead for the salvation of men. Consequently, then, Christ's Passion accomplishes man's salvation efficiently.
Reply Obj. 1: Christ's Passion in relation to His flesh is consistent with the infirmity which He took upon Himself, but in relation to the Godhead it draws infinite might from It, according to 1 Cor. 1:25: "The weakness of God is stronger than men"; because Christ's weakness, inasmuch as He is God, has a might exceeding all human power.
Reply Obj. 2: Christ's Passion, although corporeal, has yet a spiritual effect from the Godhead united: and therefore it secures its efficacy by spiritual contact—namely, by faith and the sacraments of faith, as the Apostle says (Rom. 3:25): "Whom God hath proposed to be a propitiation, through faith in His blood."
Reply Obj. 3: Christ's Passion, according as it is compared with His Godhead, operates in an efficient manner: but in so far as it is compared with the will of Christ's soul it acts in a meritorious manner: considered as being within Christ's very flesh, it acts by way of satisfaction, inasmuch as we are liberated by it from the debt of punishment; while inasmuch as we are freed from the servitude of guilt, it acts by way of redemption: but in so far as we are reconciled with God it acts by way of sacrifice, as shall be shown farther on (Q. 49). _______________________
QUESTION 49
OF THE EFFECTS OF CHRIST'S PASSION
(In Six Articles)
We have now to consider what are the effects of Christ's Passion, concerning which there are six points of inquiry:
(1) Whether we were freed from sin by Christ's Passion?
(2) Whether we were thereby delivered from the power of the devil?
(3) Whether we were freed thereby from our debt of punishment?