It's complicated. English Freemasonry does not accept atheists. Continental Freemasonry does. Swedish Freemasonry accepts only Christians.

That wasn't clear from what I read on WIkipedia etc. Freemasonry is in general quite open to different religions. But indeed belief in a 'supreme being' is required. This issue is analysed in some depth here: http://www.freemasons-freemasonry.com/atheism-freemasonry.html Interestingly the first point discussed is the suggested incapability of the convited atheist to morality.