The whole idea of "mortal sin" is unbiblical. The bible says that even one sin means one is guilty and in need of salvation.
Does the Bible teach mortal and venial sin? Does the Bible distinguish between mortal sins and venial sins?
www.gotquestions.org
When a church adds on to the bible, the adding on parts become mere
religion.
But more importantly, we are strongly and clearly commanded to not add on to the bible.
"
Do not add to what I command you and do not subtract from it, but keep the commands of the Lord your God that I give you." - Deuteronomy 4:2
"I warn everyone who hears the words of the prophecy of this book:
if anyone adds to them, God will add to him the plagues described in this book, 19
and if anyone takes away from the words of the book of this prophecy, God will take away his share in the tree of life and in the holy city, which are described in this book." - Revelation 22:18-19
You shall not add to the word that I command you, nor take from it, that you may keep the commandments of the LORD your God that I command you.
www.biblegateway.com