If you see any brother or sister commit a sin that does not lead to death, you should pray and God will give them life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that you should pray about that.
(emphasis obviously added)
In that passage, it says that
there is a sin that leads to death. What sin is he referring to here?
I'm seeking a mainstream Protestant exegesis of this scripture. I suspect that all denominations and doctrines that fall in this category will have similar understanding of this text. If I'm wrong, let me know and I'll refine my scope.