Catechism of the Catholic Church §1493 — excerpt
One who desires to obtain reconciliation with God and with the Church, must confess to a priest all the unconfessed grave sins he remembers after having carefully examined his conscience.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.