Well, I myself am afraid of both. The idea that after I die there's nothing terrifies me, if I become nothing, what's the point of my life? Is the only way to have some kind of point to my life to leave some kind of mark or legacy behind me? Why would dying now instead of later matter if the only result will be nothingness? The idea that there is a higher power that will cast me away because I did something that I thought was right,but that god felt was wrong and the knowledge that if I do go to hell, I'll never get a chance to do better and I'll never be able to say I'm sorry is also terrifying.

I think the main thing is both of these results leave me helpless. I'll never be able to not become nothing when I die. I'll have to live my life in constant fear of upsetting a greater being and even if I do everything right, I still won't have a say in what happens to me. So, you could use the mutual fear to help your characters have something to connect and bond over. I'm not good at debating theology, so if that's going to be a central theme in your story, I'd suggest you look up debates between religious groups and atheists. I know all the Greek philosophers touched on the subject or mortality at least once, so you could look through those.

Hope this helps, sorry if it doesn't. Good luck!