Well, like I said before, atheism to me is quite simple to understand. It's nothing more than saying "Hey, heard about this thing called 'God' today. Sounded bogus to me."
Really there's no more to it than that.
As for "accepting" all of the various scientific theories as "true" - I don't think most atheists I know accept various scientific theories with the same degree of unquestioned certainty that religious people approach "God".
I spent a good percentage of my life as an atheist - and I can tell you that certain theories give me more problems than others. The Big Bang is one of them. Something about that theory (and I'm not going to go into all of the details as to why) doesn't ring as true to me. It's my hunch that it's a mistake to try and find a "beginning" and an "end" to everything. While a natural thing to do - I just don't think the universe works that way. Talking about a big bang in order to satisfy that desire for there to be a beginning is really no different than believing in a God in order to satisfy the desire for there to be a beginning.
While I had a hard time accepting the big bang as being "true", that doesn't mean that I had to leap to the other option presented, which is believing that some benevolent sky fairy created everything. That seemed just as (if not more) problematic.
In short - I didn't need to *know* the answer. I was perfectly fine with hunches, suspicions of how things are, etc...and if I went my entire life without anyone giving me an answer that I was happy with or that made absolute sense to me - that was no skin off my tuchus. I've got other things to worry about. School, family, community, work, etc. Those are the real things in life that one ought to be focused on. The absolute answers to other esoteric questions are nice, but not something I need to get through life.
Most atheists I know are like that. The only thing they "know" (or rather "feel" to be true) is that God, or at least the way people describe God, just doesn't make sense to them as being true. Every other theory out there is subject to refinement, replacement, etc...as evidence becomes available. Some theories make more sense than others - and can be accepted/rejected without effecting the others. Like, believing evolution makes sense is not predicated on believing the big bang makes sense. You can have a higher degree of certainty/belief about one than the other. One isn't contingent upon the other the way that "faith" issues are.