What I can say is:
I am a Christian who believes in the Bible alone for my salvation. So I believe churches like the Catholic church and the Eastern Orthdox are unbiblical because they add extra biblical traditions like praying to the dead (like praying to Mary and the saints), and they bow down and or kiss statues (Among many other problems). So while I am strongly against Catholicism, I do agree
generally (not in detail) with the use of their terms that the Bible teaches that there are mortal sins and non-mortal sins. Mortal sin
(from a biblical perspective and not a Catholic one) is any sin that leads to spiritual death unless one confesses to Jesus, and forsakes such sins. Non-mortal sins
(from a biblical perspective and not a Catholic one) are minor faults of character that will not necessarily condemn a person. Granted, again the Catholic church is not biblical (in my view), so not everything they say is mortal sin vs. non-mortal is correct. For I believe the Catholic church adds “man made” traditions to what the Bible says.
Anyways, Jesus gives us an example of mortal sin vs. non-mortal sin in Matthew 5:22 (that many Christians today either don't see or they refuse to accept). Lets read it.
“But I say to you that everyone who is angry with his brother shall be answerable to the court; and whoever says to his brother, ‘You good-for-nothing,’ shall be answerable to the supreme court; and whoever says, ‘You fool,’ shall be guilty enough to go into the fiery hell.” (Matthew 5:22) (NASB).
The words in blue above are referring to non-mortal sins because they are punishment in earthly courts. The words in red above is in reference to a mortal sin because it is in reference to punishment in the after life in fiery hell. Granted, while I am King James Bible fan and it is my final word of authority, in this instance, the NASB translation helps to better clarify what is being said in the King James Bible (KJB). I use the terms “mortal sin” vs. “non-mortal sin” (even though it originates from Catholics) because otherwise it is difficult for other people to see what I am talking about. Terms I created for these (that is more biblical) is “grievous sin vs. errors of hidden faults,” (See: Genesis 18:20-23 and Psalms 19:12) or the sin unto death, vs. the sin not unto death (See: 1 John 5:16-17).