Catholics distinguish between mortal and venial sin because this distinction has been divinely revealed. We find it clearly stated in Sacred Scripture, where the Apostle John writes, “If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. All wrongdoing is sin, but there is sin which is not mortal” (1 John 5:16-17).
The Church teaches that mortal or deadly sin kills the life of divine grace which one receives in Baptism, cutting the sinner off from God and the Church. Three conditions must be met in order for a sin to be considered mortal: 1) it must involve serious or grave matter; 2) it must be carried out with full knowledge; and 3) it must be a free act of the will. Because of the severity involved, a mortal sin can only be forgiven through sacramental Confession. Because this sin severs one's ties with the Church, the Body of Christ, one must come to the Church (through the representative of the priest) to have those ties restored.
Conversely, venial sin is a transgression that offends God, yet without terminating one's relationship with Him. Venial sin may be forgiven through prayer (as John indicates), through the proper use of sacramentals (such as holy water), or through the reception of the sacraments.