Venial sins are less serious sins that do not cause death to the soul like mortal sins. Venial sins, while less serious in content or participation should be given strong attention because they lessen the love of God in the heart and weaken the power to resist further sin; they are still offenses against God and leave marks on the soul. Venial sins make us more prone to continue to commit sins and possibly commit mortal sins.
Venial sins can appear to be mortal sins, but fail to meet each of the three conditions for mortal sin. If a sin is grave in nature, but the person did not have full consent or sufficient knowledge that what they were doing was sinful then it would be considered a venial sin.
The Scriptures provide a differentiation between types of sins; some sins are deadly (mortal) and some are not. For this we turn to 1 John 5:16-17.
“If anyone sees his brother sinning, if the sin is not deadly, he should pray to God and he will give him life. This is only for those whose sin is not deadly. There is such a thing as deadly sin, about which I do not say that you should pray. All wrongdoing is sin, but there is sin that is not deadly.”
While not technically necessary to go to confession for venial sins, it is strongly recommended that one confesses their venial sins so as to strengthen one’s ability to resist the temptation of sin and grow in love of God. Also, while considered “light sins” when added together the light sins in aggregate can become very heavy and weigh deeply on the soul. It is best to go to confession for all sins.
Venial sin can create an unhealthy attachment to things not of God. Habitual venial sin will lead people away from God into further sin and making going to confession and seeking reconciliation with God much more challenging.