You don't have to have read the Bible to understand it, just as we can understand references to all kinds of other artifacts and myths without having read everything that was ever written about them. I doubt GL is a very big Bible reader himself, nor do I think he really believes any of it (he does use names of ancient cities found in the Bible for a few SW names though, like Endor and the Gamorians). It really is a none issue of whether or not you you believe the stories told in the Bible. The point is Raiders is clearly going with the idea that the Biblical account of the Ark is real to some degree, and that it really does have some sort of supernatural power to it. Be a pretty boring story if it was just a silly box that did nothing. Kind of like making a movie about a Philosopher's Stone that is just a rock and doesn't possess the ability to turn lead into gold, or extend your life, or make you immortal, or whatever else it has been said to be able to do over the centuries.
I don't, but then it doesn't make sense why the Nazis would want it so bad if it's just gonna kill you.
If I remember correctly, this part was clearly explained in the movie. The stories tell of the Israelites carrying the Ark into battle with them and God would help them defeat their enemies. I believe it is explained that this is what the Nazis are after. They think having the Ark in their possession will make them invincible. And actually, that is all you really need to know about the Ark to understand the story of Raiders.
I think this is why most modern movies don't really have plots either, and when they do the plot is overly simple. People relate a lot better to extensive action scenes and have a hard time digesting dialogue. Kingdom of the Crystal Skull had a really silly plot to it, but easily enough understood by a seven year old. You get convoluted crap like, "Knowledge! Knowledge was their treasure! You see Mutt! You've got to stay in school! Got that kids? Stay in school!"
EDIT: Oops, Akwat beat me to it, and explained it in half as many words and without going off on multiple tangents. That is what I get for taking my sweet time while typing. ;)