For me it would be this:
16If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life—to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that. 17All wrongdoing is sin, but there is sin that does not lead to death. (1 John 5:16-17)
It doesn't matter how many commentaries I read. None give satisfaction on this passage. I don't get it. I don't get it at all. I do not buy the fairly common explanation that this is about sin that lead to immediate death, such as in the case of Ananias and his wife Sapphira (Acts 5:1-10). That explanation has the slimmest of purchase. Apart from somebody sinning and dying, it doesn't fit nor offer any help in understanding the passage as a whole. Not to mention that since Ananias and Sapphira were summarily terminated there has been an obvious paucity of people sinning and dropping dead on the spot. With nothing new under the sun, if there are sins leading to immediate departure, you would think it would not be a rare phenomenon.
No, I don't think that's it. It is not about Ananias and Sapphira.
Sigh. It's one of those instance were I am reminded that it is not what you don't understand about the bible that should keep you awake, but what you do understand. I get that. But it's not helping. This passage has always been stuck in my craw.