Atheists, I do not want to be insulting to you so I wanted to know what the appropriate wording is. Do you call Atheism a belief system as opposed to religion?
Its not even that. There is no "system". Atheism is a label applied by people who believe in god(s) to those who don't. There is no other belief, thought or action required.
Believing in something without evidence requires faith. In fact, that is the definition of religious faith. Not believing in something because of lack of evidence does not require any faith. If it did, we would all belong to the religions of not believing in unicorns, not believing in fairies and not believing in pixies.
Buddhists are atheists, even though they have a religion. On the other hand deists don't follow a religion, even though they aren't atheists.
Humanism offers a system of values for non-believers, but not all atheists are humanists.
Some American evangelicals think atheists "believe" in evolution, but internationally more christians than atheists accept the theory of evolution. Evolution is also well supported by factual evidence, despite right wing attempts to undermine it. Disproving evolution wouldn't make atheists believe in god.
Asking what atheists believe in is a bit like asking what non-jews believe in - it covers so many different people its impossible to answer.
There is no religion, no common belief, just one disbelief.