Mortal versus Venial Sin
A serious, grave or mortal sin is the knowing and willful violation of God's law in a serious matter, for example, idolatry, adultery, murder, slander. These are all things gravely contrary to the love we owe God and, because of Him, our neighbor. As Jesus taught, when condemning even looking at a woman lustfully, sin can be both interior (choices of the will alone) or exterior (choices of the will carried into action). A man who willfully desires to fornicate, steal, murder or some other grave sin, has already seriously offended God by choosing interiorly what God has prohibited.
Mortal sin is called mortal because it is the "spiritual" death of the soul (separation from God). If we are in the state of grace it loses this supernatural life for us. If we die without repenting we will lose Him for eternity. However, by turning our hearts back to Him and receiving the Sacrament of Penance we are restored to His friendship. Catholics are not allowed to receive Communion if they have unconfessed mortal sins.
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. Additionally, something that is otherwise a mortal sin (e.g. slander) may be in a particular case only a venial sin. The person may have acted without reflection or under force of habit. Thus, not fully intending the action their guilt before God is reduced. It is always good to remember, especially those who are trying to be faithful but sometimes fall, that for mortal sin it must not only be 1) serious matter, but 2) the person must know it is serious and then 3) freely commit it.
These two categories of sin are explicitly 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.
Answered by Colin B. Donovan, STL
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