- A grave infraction of the law of God that destroys the divine life (or “sanctifying grace”) in the soul of the sinner, constituting a turn away from God. For a sin to be mortal, three conditions must be present: grave matter, full knowledge of the evil of the act, and full consent of the will.
On the other hand, venial sin is:
- Sin which does not destroy the divine life in the soul, as does mortal sin, though it diminishes and wounds it. Venial sin is the failure to observe necessary moderation, in lesser matters of the moral law, or in grave matters acting without full knowledge or complete consent.
See the difference? Mortal sins are always very serious in nature. Typically, any sin that directly breaks one of the Ten Commandments is a serious sin. If you commit such a sin with full knowledge that what you are doing is sinful and if you freely choose it (as in, nothing is forcing you to do it), then you commit a mortal sin. If the act is sinful but it doesn’t fulfill all three of the above requirements, then it is a venial sin.
Most of the sins that we commit every day are venial sins. These can be forgiven through an Act of Contrition (or some other prayer that shows God we are sorry for our sin and we desire His forgiveness) or by going to Confession. Venial sins are also forgiven whenever we receive the Eucharist. If we die with venial sins on our soul, we can still go to heaven because venial sins do not destroy the divine life within us, they simply wound it. However, that does not mean that we should have a cavalier attitude towards it. We should strive to avoid all sin, including the lesser ones. Venial sins too can be dangerous because the more we sin venially, the more likely we are to commit more serious sins. We know that sin leads to death; it is better to not even go down that road.
When a person does commit a mortal sin, then the divine life in him is destroyed. He no longer has God’s saving grace. If he were to die in this state, then he could not go to heaven. His only recourse is the Sacrament of Confession. If you have a mortal sin on your soul, then you cannot receive the Eucharist until you go to Confession.
Some people deny that there is even a difference between mortal and venial sin. They say that all sins are equal in the eyes of God. Scripture clearly refutes this (cf. 1 Jn 5:16-17) as does common sense. After all, I think we all intuitively know that killing someone is not quite the same as picking on your brother.
I hope that helps. For more on mortal and venial sin, see "Is All Sin Equal in God's Eyes?".