The distinction between mortal and venial sins is as valid today as it ever was. Mortal sin is that which is so serious as to fundamentally compromise one’s relationship with God, others and self. Venial sin involves less serious matters, and it does not compromise one’s moral integrity in the way that mortal sin does. I would estimate, as a confessor, that most sins that people confess are venial sins. Knowing whether one has committed a mortal or venial sin demands careful examination of conscience and discernment of heart and soul, as well as a determination of the degree of knowledge and consent involved in the sin.
While the Church has always held that sacramental confession is required as the normal means of reconciliation with God after mortal sin, it strongly urges that Catholics have recourse to confession for all sins. Certainly, venial sins are forgiven through means other than confession (such as fasting, prayer, almsgiving and good works); yet confession is a privileged mode for the forgiveness of venial sins.
Mortal sins are easier to identify than venial sins. In the early Church, the three major sins were apostasy, murder and adultery. It may be that most Catholics do not often commit sins that can be described as mortal. However, venial sins are very frequent, though often more vague than mortal sins. One can identify venial sins more readily through an examination of conscience concerning the good deeds that one has failed to do. Through an examination of conscience, one can discern the ways in which one has failed morally.
A Catholic should never feel reluctant to go to confession for the forgiveness of venial sins. Nor should a priest ever make someone feel awkward when he or she has no mortal sins to confess. One of the primary roles of a confessor is to assist people in the identification of their sins. For that reason, if one is not sure what one should confess, then the assistance of the priest should be sought.