Objection 1. It would seem that we were not delivered from sin through Christ's Passion. For to deliver from sin belongs to God alone, according to Isaiah 43:25: "I am He who blot out your iniquities for My own sake." But Christ did not suffer as God, but as man. Therefore Christ's Passion did not free us from sin.
Objection 2. Further, what is corporeal does not act upon what is spiritual. But Christ's Passion is corporeal, whereas sin exists in the soul, which is a spiritual creature. Therefore Christ's Passion could not cleanse us from sin.
Objection 3. Further, one cannot be purged from a sin not yet committed, but which shall be committed hereafter. Since, then, many sins have been committed since Christ's death, and are being committed daily, it seems that we were not delivered from sin by Christ's death.
Objection 4. Further, given an efficient cause, nothing else is required for producing the effect. But other things besides are required for the forgiveness of sins, such as baptism and penance. Consequently it seems that Christ's Passion is not the sufficient cause of the forgiveness of sins.
Objection 5. Further, it is written (Proverbs 10:12): "Charity covereth all sins"; and (Proverbs 15:27): "By mercy and faith, sins are purged away." But there are many other things of which we have faith, and which excite charity. Therefore Christ's Passion is not the proper cause of the forgiveness of sins.
On the contrary, It is written (Apocalypse 1:5): "He loved us, and washed us from our sins in His own blood."
I answer that, Christ's Passion is the proper cause of the forgiveness of sins in three ways. First of all, by way of exciting our charity, because, as the Apostle says (Romans 5:8): "God commendeth His charity towards us: because when as yet we were sinners, according to the time, Christ died for us." But it is by charity that we procure pardon of our sins, according to Luke 7:47: "Many sins are forgiven her because she hath loved much." Secondly, Christ's Passion causes forgiveness of sins by way of redemption. For since He is our head, then, by the Passion which He endured from love and obedience, He delivered us as His members from our sins, as by the price of His Passion: in the same way as if a man by the good industry of his hands were to redeem himself from a sin committed with his feet. For, just as the natural body is one though made up of diverse members, so the whole Church, Christ's mystic body, is reckoned as one person with its head, which is Christ. Thirdly, by way of efficiency, inasmuch as Christ's flesh, wherein He endured the Passion, is the instrument of the Godhead, so that His sufferings and actions operate with Divine power for expelling sin.
Reply to Objection 1. Although Christ did not suffer as God, nevertheless His flesh is the instrument of the Godhead; and hence it is that His Passion has a kind of Divine Power of casting out sin, as was said above.
Reply to Objection 2. Although Christ's Passion is corporeal, still it derives a kind of spiritual energy from the Godhead, to which the flesh is united as an instrument: and according to this power Christ's Passion is the cause of the forgiveness of sins.
Reply to Objection 3. Christ by His Passion delivered us from our sins causally--that is, by setting up the cause of our deliverance, from which cause all sins whatsoever, past, present, or to come, could be forgiven: just as if a doctor were to prepare a medicine by which all sicknesses can be cured even in future.
Reply to Objection 4. As stated above, since Christ's Passion preceded, as a kind of universal cause of the forgiveness of sins, it needs to be applied to each individual for the cleansing of personal sins. Now this is done by baptism and penance and the other sacraments, which derive their power from Christ's Passion, as shall be shown later (62, 5).
Reply to Objection 5. Christ's Passion is applied to us even through faith, that we may share in its fruits, according to Romans 3:25: "Whom God hath proposed to be a propitiation, through faith in His blood." But the faith through which we are cleansed from sin is not "lifeless faith," which can exist even with sin, but "faith living" through charity; that thus Christ's Passion may be applied to us, not only as to our minds, but also as to our hearts. And even in this way sins are forgiven through the power of the Passion of Christ.
Source: Summa Theologica (New Advent)