341. The third commandment of the Church is "To confess our sins at least once a year". The Fourth Council of Lateran enacted this law (n. 260) as follows: "All of the faithful of both sexes, after they have arrived at the age of discretion, shall once a year faithfully confess all their sins privately to the proper priest" (can. 21). The sins here spoken of are mortal sins; for these alone need be confessed, as is explained by the Council of Trent (Sess. 14). The proper priest is any priest approved by the Bishop of the diocese for the office of hearing confessions. The age of discretion is that at which a child becomes capable of understanding the evil of mortal sin; which is usually considered to be the age of seven years.
342. To receive this Sacrament worthily the penitent must approach it with true sorrow for sin and a firm purpose of amendment. He must confess all the grievous sins which he has committed since his last worthy confession; or, if this be his first confession, since his Baptism. After the confession, he must obtain absolution, and fulfil the penance enjoined. By way of preparation for confession, he should, as far as circumstances allow, examine his conscience with sufficient care to make it probable that he recalls all the sins which he is obliged to confess. The common and commendable practice of daily examination of conscience greatly facilitates the task when the time comes to prepare for confession.
343. Sorrow for all mortal sins committed, including the purpose to avoid them for the future, -- else the sorrow were not sincere, -- is so necessary, that without it not even one sin can be forgiven. This sorrow should be: a) Sincere, -- the Council of Trent calls it: "A sorrow of the soul, and detestation of sins committed, with a purpose of sinning no more"; b) Supernatural, that is, conceived for a motive which is apprehended by faith such as the fear of God's punishment, the loss of Heaven, God's hatred of sin, His goodness, His benefits, the sufferings which Christ endured for our sins, etc. c) Sovereign, estimating the evil of sin as the greatest evil; d) Universal, extending to all one's mortal sins.
If we have committed no mortal sin, we should be sorry for at least one of the venial sins confessed, or we may confess some sin already forgiven for which we still grieve; for sorrow is a necessary condition to receive absolution worthily.
If our sorrow for sin flows from the perfect love of God, that is from our love of God for His own sake (n. 308), our sorrow is then called perfect contrition. From the moment we conceive it, we obtain pardon of our sins, provided we be willing to confess them duly; for perfect love of God and mortal sin cannot exist together. It is therefore an excellent practice frequently to make acts of perfect contrition. If our sorrow flows from a less perfect motive, say from fear of punishment or love of reward, it is imperfect contrition, also called attrition; and it is not sufficient to obtain pardon of sin without the absolution of the priest.
344. The purpose of amendment must be: a) Firm, so that we can say, not "I would like to avoid mortal sins", but "I am determined to avoid them". We know our weakness; but we trust in God's help, for which we are resolved to pray; b) Universal, extending to the avoidance of all mortal sins generally; c) Efficacious, comprising a firm resolve to use the necessary means to avoid sin; in particular to avoid the proximate occasions of sins, those namely which are likely to lead us to a serious fall.
345. The confession must be: a) Sincere and humble, since we make it to the representative of God; we should make it to accuse, not to excuse ourselves. Still sins should not be exaggerated, nor doubtful ones confessed as certain. c) Entire, embracing all the mortal sins which the penitent is conscious of having committed since his Baptism, and which he has not yet confessed and been absolved from. If he were voluntarily to omit even one of these, when it is morally possible for him to confess them all, the Sacrament would be unworthily received, and would take away no sin; but there would be added to his sins the guilt of sacrilege. Such a confession would have to be repeated, and the sacrilege confessed, before absolution could be obtained. If however a mortal sin were inculpably omitted, all the sins would be pardoned; but the one omitted would have to be confessed, as soon as convenient, or in the next confession. With each mortal sin, those circumstances must be explained which change its species; also the number of sins committed in each species, as far as it can be known
(n. 260). After receiving absolution, there still remains the task incumbent on the penitent to perform the penance imposed by the priest, as was explained before (n. 262).