As a result of Original Sin, human nature is weakened. Baptism, by imparting the life of Christ’s grace, takes away Original Sin and turns us back toward God. The consequences of this weakness and the inclination to evil persist, and we often commit personal or actual sin.
Actual sin is sin which people commit. There are two kinds of actual sin, mortal and venial.
Mortal sin is a deadly offense against God, so horrible that it destroys the life of grace in the soul. Three simultaneous conditions must be fulfilled for a mortal sin: 1) the act must be something very serious; 2) the person must have sufficient understanding of what is being done; 3) the person must have sufficient freedom of the will.
Venial sins are slight sins. They do not break our friendship with God, although they injure it. They involve disobedience of the law of God in slight (venial) matters. If we gossip and destroy a person’s reputation it would be a mortal sin. However, normally gossip is about trivial matters and only venially sinful. The person may have acted without reflection or underforce of habit. Thus, not fully intending the action their guilt before God is reduced.
These two categories of sin are explicit to be found in Sacred Scripture. In the Old Covenant, there were sins that merited the death penalty and sins that could be expiated by an offering. This Law was a teacher that prepared the way for the faith (Gal. 3:24). In the New Covenant, these material categories are replaced by spiritual ones, natural death by eternal death. There are thus daily faults for which we must daily ask forgiveness (Mt. 6:12), for even the “just man falls seven times a day” (Prov. 24:16), and mortal faults that separate the sinner from God (1 Cor. 6:9-10) for all eternity.