According to Roman Catholicism, a venial sin or forgiveable sin is a lesser sin that does not result in a complete separation from God and eternal damnation in hell. A venial sin involves a partial loss of grace from God.
A venial sin meets at least one of the following criteria:
1. It does not concern a grave matter,
2. It is not committed with full knowledge, or
3. It is not committed with both deliberate and complete consent.
As the above criteria are the three criteria for mortal sin stated negatively, a sin which met none of these extenuating conditions would necessarily be considered mortal.
Each venial sin that one commits adds to the penance that one must do. Penance left undone during life converts to punishment in purgatory. A venial sin can be left unconfessed, though – so long as there is some purpose of amendment – it is helpful to confess, for one receives from the sacrament grace to help overcome it.
Venial sins usually remain venial no matter how many one commits. They cannot add up to collectively constitute a mortal sin. However, there are cases where repeat offences may become a grave matter. For instance, if one were to steal small amounts of property from a particular person, over time one would have stolen enough that it would become a serious theft from that person.
In all this, one ought not to take venial sin lightly, especially when committed deliberately. No one without a special grace can avoid even semi-deliberate venial sins entirely – according to the definition of Trent. But one must, even to avoid mortal sins, seek as far as possible to overcome venial sin; for though a number of venial sins do not themselves add up to a mortal sin, each venial sin weakens the will further, and the more willing one becomes in allowing such falls, the more one is inclined towards, and will inevitably fall into – if one continues along this path – mortal sin.