Venial sin is not deadly, but is a movement towards a mortal from temptation, for these sins we can pray directly to God and repent from falling into temptation. Ones penitential rite in the beginning of Mass is one way of praying for our venial sins, which John teaches about sins which are not deadly we can pray to God.
Mortal sin is deadly. Mortal sin is a willful act that breaks the divine law (Decalogue) and separates oneself from God’s graces. A perfect blood sacrifice pleasing to God, confession and penance, atones for our mortal sin which leads to eternal death and separation from God’s divine life.
1 John 5:16 If you see any brother or sister commit a sin that does not lead to death, you should pray and God will give them life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that you should pray about that. 17 All wrongdoing is sin, and there is sin that does not lead to death.