MORTAL SIN. This expression is not found in this form in the Bible, but the Scripture makes it clear that sin in itself, and therefore every manifestation of a depraved nature, issues unto death (Rom 6:23, etc.).
In Roman Catholic theology, mortal sin is contrasted with “venial” sin which is seen as less aggravated and therefore much less damaging to the soul than mortal sin. This distinction is not explicitly asserted in Scripture.
In 1 John 5:16, 17, the apostle distinguishes between “sin unto death” and “sin not unto death.” Inasmuch as he directs that no prayer should be offered for the former, this passage has been quite naturally paralleled to Matthew 12:31, 32; Hebrews 6:4-6. It is not certain that all three of these passages refer to the same kind of transgression; but if this be so, the sin in view would appear to be the hardening of the heart against the offer of the divine light in its most unmistakable form. This interpretation commends itself to the evangelical scholar at many points.