In order for the sacrament of Penance to be validly celebrated, the penitent must confess all mortal sins.
If the penitent knowingly withholds any mortal or serious sin, then the confession is invalid and the penitent incurs another sin: sacrilege. If the penitent has truly and honestly forgotten to mention a mortal sin in Confession, however, then upon absolution all their sins are forgiven.
You are only held accountable for that which you know. It is common practice for the penitent to say at the end of confession, “…these and all my sins I have forgotten since my last confession, I ask for absolution and penance.”
What is a mortal sin? Objectively, it is grievous matter—something seriously wrong that is or leads to a severe violation of God’s commandments. Subjectively, there are two other conditions for a sin to be mortal—knowledge and free will. The person must know that the sin is a grave matter and freely do it anyway. All three conditions have to be present in order for a sin to be mortal. Otherwise, the sin is venial or less serious.
One does not have to go through the sacrament of Penance to confess venial sins. A good act of contrition—the taking of Holy Water with the consciousness of one’s venial sins, asking pardon, and finally, Holy Mass itself, with a contrite and penitential spirit—will remove venial sins. Catholics are still encouraged to go to Confession even if only for venial sins, since these sins can wear the soul down and make it vulnerable to mortal sins.
Think of a venial sin as if it were a cold. When left untreated, a cold can turn into bronchitis or pneumonia. The grace conferred through the sacrament of Penance is protective and will help the penitent to be more aware of his surroundings, temptations, and things that need to be changed. The grace of the sacrament—God’s healing life within—will help the penitent to avoid the near occasion of sin or temptation in the future. In an analogous way, the grace of the sacrament becomes a vitamin to build up the soul in order to help it resist sin in the future.
Sometimes people go to the sacrament of Reconciliation in order to make what is known as a general confession. A general confession is going over one’s whole past. It is true that once a sin is forgiven in the sacrament of Penance, it does not have to ever be brought up again. At times, before a major change in one’s life, a devotional or general confession is good for the spiritual life. It could take place before one gets married, is ordained, makes a major career change, or after a retreat. Thinking about where one has been, is now, and wants to go, helps the person to reflect upon past mistakes in order not to commit them in the future.
Finally, people go to the sacrament of Penance when they are dealing with a bad habit that they are trying to break. Though the person is not in mortal sin because free will is reduced, he or she still has the obligation to change. The sacrament of Penance, with its healing balm and strengthening grace, will help the penitent to “kick the habit” and replace it with a good one. The priest offers advice and encouragement to the penitent, which also aids in the healing process.
To help the penitent, a series of pamphlets entitled “An Examination of Conscience” takes the Ten Commandments and all their refinements and puts them into modern language. They also reflect on the six commandments of the Church and the seven capital, or deadly, sins which are also listed as vices. The examination of one’s conscience must take place prior to the sacrament. It helps penitents to be truly honest with themselves.
The six commandments, or precepts of the Church, are specific duties of Catholics: to keep holy the day of the Lord’s resurrection and holy days of obligation; to lead a sacramental life by receiving Holy Communion and Penance regularly; to study Catholic teaching in preparation for the sacrament of Confirmation, then to be confirmed and continue to study and advance the cause of Christ; to observe the marriage laws of the Church and to give religious training to one’s children; to strengthen and support the Church by stewardship of time, talent, and treasure; and finally, to do penance, including abstaining from meat and fasting from food on the appointed days.
The seven capital sins are pride, covetousness, lust, anger, gluttony, envy, and sloth. These deadly sins are always at the root of or the motivation behind the violation of God’s commandments. They are fodder for confession because in confessing, the penitents becomes aware of what make them sin.