Actually, since science deals with evidence, it is by definition atheist. It does not claim that there is no god (same as science has never claimed there is no Santa Claus); it merely contents itself in not including the idea of gods until there is a reason to. Science, then, is not atheistic in the sense of "there is no god", but rather it is atheistic in the sense that it does not include gods. And that's the way most atheists think, too. Very few would say "there is no god", unless they've been disillusioned in a former faith. The atheist (modern or otherwise) thinks, "I don't believe in gods as there is no evidence". Science looks for naturalistic explanations for everything.
As for probability:
Without knowing the actual odds, you have a 50/50 chance of guessing correctly if there are only two options. But that doesn't translate into a 50% chance of it being correct. For example, if I have a box with 8 blue marbles and 12 red ones, you have a 50% chance of guessing which marble will be selected next if you don't know how many of each there are. However, the actual odds are 60/40 in favour of red.