CCC 1456 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.”¹
When Christ’s faithful strive to confess all the sins that they can remember, they undoubtedly place all of them before the divine mercy for pardon. But those who fail to do so and knowingly withhold some, place nothing before the divine goodness for remission through the mediation of the priest, “for if the sick person is too ashamed to show his wound to the doctor, the medicine cannot heal what it does not know.”²

1 Council of Trent (1551): DS 1680 (ND 1626); cf. Ex 20:17; Mt 5:28.
2 Council of Trent (1551): DS 1680 (ND 1626); cf. St. Jerome, In Eccl. 10, 11: PL 23:1096.