According to Catholic belief a venial sin is an offense against the Laws of God, not grave enough to cause a complete loss of sanctifying grace.
Venial sin is seen as an illness of the soul rather than its death. Because the soul committing a venial sin falls short of perfection but is still united with God and capable of charity, it does not receive eternal damnation, as we find with the unforgiven mortal sin.
Instead, venial sins merit temporary punishments which purify the soul so as to prepare it for everlasting life in heaven.
The excellent – if old – Catholic Encyclopedia (1912) outlines some scriptural references held by some to support the classification of venial vs. mortal sin.
The distinction between mortal and venial sin is set forth in Scripture. From St. John (1 John 5:16-17) it is clear there are some sins “unto death” and some sins not “unto death”, i.e. mortal and venial. The classic text for the distinction of mortal and venial sin is that of St. Paul (1 Corinthians 3:8-15), where he explains in detail the distinction between mortal and venial sin.¹
More recently, Wikipedia adds:²
A venial sin meets at least one of the following criteria:
- It does not concern a “grave matter”,
- It is not committed with full knowledge, or
- It is not committed with both deliberate and complete consent.
¹ O’Neil, A.C. (1912). Sin. In The Catholic Encyclopedia. New York: Robert Appleton Company. Retrieved October 11, 2008 from New Advent: http://www.newadvent.org/cathen/14004b.htm