It has been objected that the
long standing teaching of the
Church of a distinction between "mortal" and
"venial" sin
was an artificial distinction.
But it was the Apostle John,
Evangelist and author of three
epistles as well as the Book
of Revelation, from whom the
church took that distinction
among sins.
1 John 5:16-17
If anyone sees his brother sinning, if the sin is not
deadly (venial), 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 (mortal), about
which I do not say that you should pray. All wrongdoing
is sin, but there is sin that is not deadly.