The manifestation of one’s own actual sins, committed after Baptism, to a priest, in order to obtain their forgiveness in the Sacrament of Penance.
Confession is Scriptural
Confession was constituted an essential part of this sacrament by Christ Himself, when He said: “Whose sins you shall forgive, they are forgiven them; and whose sins you shall retain, they are retained” (John, 20). By these words Christ established the Sacrament of Penance as a judicial process, and its ministers, the Apostles and their successors, as judges, with the right and the duty to pass judgment on those who have sinned after Baptism.
Confession is Historical
Now, in order to perform this office properly, a priest must have a knowledge of the penitent’s transgressions, which can be obtained only from a sincere confession of the penitent himself. This self-accusation should include all mortal sins not yet properly confessed and forgiven, since they constitute the necessary matter of this sacramental judgment. That confession was regarded even from the first ages of Christianity as a necessary condition for the pardon of sins committed after Baptism is attested in the writings of the early Fathers, e.g., Tertullian, Origen, Saint Cyprian.
In the first centuries confession was often public; but private confession was also in use, especially for occult sins. Since the priest’s juridical office demands that he have a complete knowledge of the penitent’s conscience, the latter should confess both the nature and the number of his mortal sins. However, when such particularization is impossible, e.g., in the case of a regiment of soldiers on their way to battle, a general acknowledgment of sin is sufficient for a sacramental confession; but one who has received absolution in such circumstances is obliged to confess his mortal sins in detail the next time he approaches sacramental confession.