I John 5:16-18:
If anyone sees his brother committing a sin that is not a deadly sin, he will ask, and God will give him life for those whose sin is not deadly. There is sin which is deadly; I do not say one is to pray for that. All wrongdoing is sin, but there is sin which is not deadly. We know that anyone born of God does not sin, but He who is born of God keeps him, and the evil one does not touch him.
Three points:
1. These verses cannot be any plainer that there is such a thing as "deadly sin" and "sin which is not deadly." That is precisely what the Church means by mortal (sin unto death) and venial (sin not unto death) sin.
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Where Do We Find Purgatory in the Bible?
Devotion to the souls in Purgatory contains in itself all the works of mercy, which supernaturalized by a spirit of faith, should merit us Heaven. de Sales