Summa Part III, Q 48; art. 5
I answer that, For someone to redeem, two things are required--namely, the act of paying and the price paid. For if inredeeming something a man pays a price which is not his own, but another's, he is not said to be the chief redeemer, but rather the other is, whose price it is. Now Christ's blood or His bodily life, which "is in the blood," is the price of our redemption(Leviticus 17:11-14), and that life He paid. Hence both of these belong immediately to Christ as man; but to the Trinity as to the first and remote cause, to whom Christ's life belonged as to its first author, and from whom Christ received the inspiration of suffering for us. Consequently it is proper to Christ as man to be the Redeemer immediately; although the redemption may be ascribed to the whole Trinity as its first cause.
Reply to Objection 1. A gloss explains the text thus: "Thou, O Lord God of Truth, hast redeemed me in Christ, crying out, 'Lord, into Thy hands I commend my spirit.'" And so redemption belongs immediately to the Man-Christ, but principally to God.
Reply to Objection 2. The Man-Christ paid the price of our redemption immediately, but at the command of the Father as the original author.
Reply to Objection 3. The sufferings of the saints are beneficial to the Church, as by way, not of redemption, but of example and exhortation, according to 2 Corinthians 1:6: "Whether we be in tribulation, it is for your exhortation and salvation.
And Article 6
There is a twofold efficient agency--namely, the principal and the instrumental. Now the principal efficient causeof man's salvation is God. But since Christ's humanity is the "instrument of the Godhead," as stated above (Question 43, Article 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 to Objection 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 Corinthians 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 to Objection 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 (Romans 3:25): "Whom God hath proposed to be a propitiation, through faith in His blood."
Reply to Objection 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, itacts 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 (49)