I was never a part of a group that believed in hell (hope that doesn't give me away...), so I can't speak for that.
But, if you relax the rigor and you're willing to settle for what you personally qualify as overwhelming circumstantial evidence, it's not too hard to meet that level of confidence (the Anthropic Principle is very powerful). These people are often True Believers.
It's a whole different mindset, and worth spending the effort to wrap your head around. I'd bet it's a sufficiently different frame of mind that you'll have a hard time relating. It's easy to just gloss it over as a fool's thinking, but it really can be quite complex and deep (after all, without that moral vacuum, there's a lot of core axioms to keep straight). And that's not bad; just be warned that it can be a bit of a memetic virus, but it's certainly not too hard to quarantine. Usually at a very low level there's an assumption made about the way things are, and enough evidence is brought in to make that assumption feel reasonable; that's not science but rather more of a judicial argument. If it feels a bit boot-strappy, you're a bit on track. Typically once the mindset is settled, that rationalizes the initial bootstrap; I think often those who preach worry that many will not be exposed to the needed bootstrap, and feel a moral imperative to help others through that process. And that's how you get the evangelism. (Clearly there's a gross amount of glossing over here :)
And it'll help a lot in pleasantly/tactfully dealing with such individuals. People are just folk, and most are sincere. Just not always as... rigorous as folk like yourself.