It is the Passion of Christ that has freed us from sin

He hath loved us and washed us from our sins in his own blood.
Ap. 1, 5

The Passion of Christ is the proper cause of the remission of our sins, and that in three ways.

1. Because it provokes us to love God. St. Paul says, God commendeth his charity towards us; because when as yet we were sinners Christ died for us (Rm. 5, 8).

Through charity we obtain forgiveness for sin, as it says in the gospel, Many sins are forgiven her because she hath loved much (Lc. 7, 47).

2. The Passion of Christ is the cause of the forgiveness of sins because it is an act of redemption. Since Christ is himself our head, he has, by his own Passion — undertaken from love and obedience — delivered us his members from our sins, as it were at the price of his Passion. Just as a man might by some act of goodness done with his hands buy himself off for a wrong thing he had done with his feet. For as man's natural body is a unity, made up of different limbs, so the whole Church, which is the mystical body of Christ, is reckoned as a single person with its own head, and this head is Christ.

3. The Passion of Christ was a thing equal to its task. For the human nature through which Christ suffered his Passion is the instrument of His divine nature. Whence all the actions and all the sufferings of that human nature wrought to drive out sin, are wrought by a power that is divine.

Christ, in His Passion, delivered us from our sins in a causal way, that is to say, he set up for us a thing which would be a cause of our emancipation, a thing whereby any sin might at any time be remitted, whether committed now, or in times gone by, or in time to come: much as a physician might make a medicine from which all who are sick may be healed, even those sick in the years yet to come.

But since what gives the Passion of Christ its excellence is the fact that it is the universal cause of the forgiveness of sins, it is necessary that we each of us ourselves make use of it for the forgiveness of our own particular sins. This is done through Baptism, Penance and the other sacraments, whose power derives from the Passion of Christ.

By faith also we make use of the Passion of Christ, in order to receive its fruits, as St. Paul says, Christ Jesus, whom God hath proposed to be a propitiation, through faith in his blood (Rom. iii 25). But the faith by which we are cleansed from sin is not that faith which can exist side by side with sin — the faith called formless — but faith formed, that is to say, faith made alive by charity. So that the Passion of Christ is not through faith applied merely to our understanding but also to our will. Again, it is from the power of the Passion of Christ that the sins are forgiven which are forgiven by faith in this way.

(3 49 1)