No. 1420 and 1421, page 357 of the Catechism of the Catholic Church: The Celebration of the Christian Mystery; Chapter Two - The Sacraments of Healing; Libreria Editrice Vaticana: Urbi Et Orbi Communications:
Through the sacraments of Christian initian, man receives the new life of Christ. Now we carry this life "in earthen vessels," and it remains "hidden with Christ in God." We are still in our "earthly tent," subject to suffering, illness, and death. This new life as a child of God can be weakened and even lost by sin.
"Those who approach the sacrament of Penance obtain pardon from God's mercy for the offense committed aginst him, and are, at the same time, reconciled with the Church which they have wounded by their sins and which by charity, by example, and by prayer labors for their conversion."
From the Baltimore Catechism No. 3; Benziger Brothers, Inc. and Tan Books and Publishers, Inc. Nos. 721, 722, and 723, page 149 and 150.
Q. What is the Sacrament of Penance?
A. Penance is a Sacrament in which the sins committed after Baptism are forgiven.
Q. Has the word Penance any other meaning?
A. The word Penance has other meanings. It means also those punishments we inflict upon ourselves as a means of atoning for our past sins; it means likewise that disposition of the heart in which we detest and bewail our sins because they were offensive to God.
Q. How does the institution of the Sacrament of Penance show the goodness of Our Lord?
A. The institution of the Sacrament of Penance shows the goodness of Our Lord, because having once saved us through Baptism, He might have left us to perish if we again committed sin.