Objection 1. It seems that the sacraments of the New Law do not derive their power from Christ's Passion. For the power of the sacraments is in the causing of grace which is the principle of spiritual life in the soul. But as Augustine says (Tract. xix in Joan.): "The Word, as He was in the beginning with God, quickens souls; as He was made flesh, quickens bodies." Since, therefore, Christ's Passion pertains to the Word as made flesh, it seems that it cannot cause the power of the sacraments.
Objection 2. Further, the power of the sacraments seems to depend on faith. for as Augustine says (Tract. lxxx in Joan.), the Divine Word perfects the sacrament "not because it is spoken, but because it is believed." But our faith regards not only Christ's Passion, but also the other mysteries of His humanity, and in a yet higher measure, His Godhead. Therefore it seems that the power of the sacraments is not due specially to Christ's Passion.
Objection 3. Further, the sacraments are ordained unto man's justification, according to 1 Corinthians 6:11: "You are washed... you are justified." Now justification is ascribed to the Resurrection, according to Romans 4:25: "(Who) rose again for our justification." Therefore it seems that the sacraments derive their power from Christ's Resurrection rather than from His Passion.
On the contrary, on Romans 5:14: "After the similitude of the transgression of Adam," etc., the gloss says: "From the side of Christ asleep on the Cross flowed the sacraments which brought salvation to the Church." Consequently, it seems that the sacraments derive their power from Christ's Passion.
I answer that, As stated above (Article 1) a sacrament in causing grace works after the manner of an instrument. Now an instrument is twofold. the one, separate, as a stick, for instance; the other, united, as a hand. Moreover, the separate instrument is moved by means of the united instrument, as a stick by the hand. Now the principal efficient cause of grace is God Himself, in comparison with Whom Christ's humanity is as a united instrument, whereas the sacrament is as a separate instrument. Consequently, the saving power must needs be derived by the sacraments from Christ's Godhead through His humanity.
Now sacramental grace seems to be ordained principally to two things: namely, to take away the defects consequent on past sins, in so far as they are transitory in act, but endure in guilt; and, further, to perfect the soul in things pertaining to Divine Worship in regard to the Christian Religion. But it is manifest from what has been stated above (48, 1,2,6; 49, 1,3) that Christ delivered us from our sins principally through His Passion, not only by way of efficiency and merit, but also by way of satisfaction. Likewise by His Passion He inaugurated the Rites of the Christian Religion by offering "Himself--an oblation and a sacrifice to God" (Ephesians 5:2). Wherefore it is manifest that the sacraments of the Church derive their power specially from Christ's Passion, the virtue of which is in a manner united to us by our receiving the sacraments. It was in sign of this that from the side of Christ hanging on the Cross there flowed water and blood, the former of which belongs to Baptism, the latter to the Eucharist, which are the principal sacraments.
Reply to Objection 1. The Word, forasmuch as He was in the beginning with God, quickens souls as principal agent; but His flesh, and the mysteries accomplished therein, are as instrumental causes in the process of giving life to the soul: while in giving life to the body they act not only as instrumental causes, but also to a certain extent as exemplars, as we stated above (56, 1, ad 3).
Reply to Objection 2. Christ dwells in us "by faith" (Ephesians 3:17). Consequently, by faith Christ's power is united to us. Now the power of blotting out sin belongs in a special way to His Passion. And therefore men are delivered from sin especially by faith in His Passion, according to Romans 3:25: "Whom God hath proposed to be a propitiation through faith in His Blood." Therefore the power of the sacraments which is ordained unto the remission of sins is derived principally from faith in Christ's Passion.
Reply to Objection 3. Justification is ascribed to the Resurrection by reason of the term "whither," which is newness of life through grace. But it is ascribed to the Passion by reason of the term "whence," i.e. in regard to the forgiveness of sin.
Source: Summa Theologica (New Advent)