[SkyPilot]

The Bible does tell us to confess our sins to God and to each other, but the terms and classification of mortal vs. venial sin is not in Scripture, but made up by the Roman church. They will use certain versus to claim these two classifications are referred to by God's word, but it is a very obscure and murky argument at best.

I once asked a Catholic priest how I could be assured salvation and heaven. He stammered for a bit before saying, "Well, that is a Great Mystery."

[Campion]

but the terms and classification of mortal vs. venial sin is not in Scripture, but made up by the Roman church

Don't think so.

If anyone sees his brother sinning a sin which does not lead to death, he will ask, and He will give him life for those who commit sin not leading to death. There is sin leading to death. I do not say that he should pray about that. 17 All unrighteousness is sin, and there is sin not leading to death. -- 1 John 5:16-17, NKJV

"Leading to death" is pros thanatos in the Greek. It's absolutely reasonable to translate "pros thanatos" as "mortal".