While it's certainly true that theists can 'believe" in evolution, but it's not strictly true that atheists likewise necessarily "believe" in evolution because anyone familiar with scientific empiricism knows that there's not much room for belief in science. What we say about scientific theories is that they are the best current explanation of a given phenomenon. Science is the process of working toward ever more adequate explanations of how things work. There is no value to mere "belief."

It is this fundamental difference between facile belief (which requires no intellectual effort, and is thus highly error-prone) and scientific endeavor, which is a never-ending process. Theists are content with mere belief; atheists generally - but not always - tend to be more intellectually rigorous.