No. 1455 and 1456, page 365 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:
Confession to a priest is an essential part of the sacrament of Penance: "All mortal sins of which penitents after a diligent self-examination are conscious must be recounted by them in confession, even if they are most secret and have been committed against the last two precepts of the Decalogue; for these sins sometimes wound the soul more grievously and are more dangerous than those which are committed openly. (Council of Trent: DS 1676)
From the Baltimore Catechism No. 3; Benziger Brothers, Inc. and Tan Books and Publishers, Inc. Nos. 749, 751 and 752, page 157, and 796 and 797, pages 167 and 168.
Q. What is the examination of conscience?
A. The examination of conscience is an earnest effort to recall to mind all the sins we have committed since our last worthy confession.
Q. How can we make a good examination of conscience?
A. We can make a good examination of conscience by calling to memory the commandments of God, the precepts of the Church, the seven capital sins, and the particular duties of our state in life, to find out the sins we have committed.
Q. What should we do before beginning the examination of conscience?
A. Before beginning the examination of conscience we should pray to God to give us light to know our sins and grace to detest them.
Q. How is concealing a sin telling a lie to the Holy Ghost?
A. Concealing a sin is telling a lie to the Holy Ghost, because he who conceals the sin declares in confession to God and the priest that he committed no sins but what he has confessed, while the Holy Ghost, the Spirit of Truth, saw him committing the sin he now conceals and still sees it in his soul while he denies it.
Q. Why is it foolish to conceal sins in confession?
A. It is foolish to conceal sins in confession: (1) Because we thereby make our spiritual condition worse; (2) We must tell the sin sometime if we ever hope to be saved; (3) It will be made known on the day of judgment, before the world, whether we conceal it now or confess it. A.