Hmmm..., well my Bible doesn't mention anything about mortal or venial sins so I assume this is all part of the tradition of Catholicism.
I'm certain that my brand of Christianity looks odd to Catholics, just as the practice of Catholicism looks odd to me. Maybe we should resolve to do as St Paul admonishes in 1 Corinthians 10 and seek the good of all-- "Do not cause anyone to stumble, whether Jews, Greeks, or the church of God--"
Your Bible doesn't have the 10 Commandments!!??