Mortal and Venial SinThe Problem: Fundamentalists and evangelicals often claim that all sins are equally heinous before God - stealing a penny is as bad as slaughtering an entire household.
The Truth: There is sin which destroys our life in Christ. This sin is called mortal sin, for by it we are cut off from the source of grace, and are dead to Christ, having rejected His salvific grace in us. We are only brought back to life in Christ, and brought back in the ability to share in the sacramental life of grace, through the sacrament of Confession, in which Christ revives the life of grace within us through a gift of grace, cleansing our soul of sin. There is also sin which does not destroy our life in Christ, but wounds it, and leads to a partial separation from His Body. This sin is called venial sin. We can continue to partake of the life of Christ poured out in the sacraments when we are in this state, but are urged to avail ourselves of the cleansing grace of Confession at our earliest opportunity.
1 Jn 5:16-17 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. 17 All wrongdoing is sin, but there is sin which is not mortal.
1 John 5:16 is often the easiest way to make a sola scriptura enthusiast sit up and take notice. For some reason, even people who have read Scripture cover to cover twelve times simply don't remember seeing this verse until it is pointed out to them.
Gal 5:19-21 Now the works of the flesh are plain: fornication, impurity, licentiousness, 20 idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit,
Eph 5:5 Be sure of this, that no fornicator or impure man, or one who is covetous (that is, an idolater), has any inheritance in the kingdom of Christ and of God.
1 Cor 6:9-10 Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither the immoral, nor idolaters, nor adulterers, nor sexual perverts, 10 nor thieves, nor the greedy, nor drunkards, nor revilers, nor robbers will inherit the kingdom of God.
There are sins which can lose us our salvation. Paul tells us which sins those are. They are mortal sin. Not all sins are mortal, nor are all sins venial, else Paul would not bother to distinguish them.
Early Christians' comments on Mortal Sin