I do hope you’re not referring to me! I’m the kind of lifelong unbeliever who doesn’t need to talk about my lack of belief or challenge others regarding theirs. I don’t proselytise or assert my views are correct, however if I’m told by others that I’m wrong to not believe in god, or when they smugly tell me that they have their place booked in heaven whilst I’m destined for hell, I may just smirk a little at their credulity.

Where is the line between "smug" self-assurance and regular old self-assurance?

That would be arrogance, which is never a good look!

Self-assurance is uff-da fine (how to talk Minni-soooaatan), CERTAINTY is not a good look.