Perhaps religion is too strong a word to describe groups of atheists when they come together. Perhaps ideology would be a better application.
Ideology: the set of ideas and beliefs of a group or political party
The singular atheist by himself has no religion and is only defined by his non-belief in a deity. Like all human groups, however, when atheists come together and form a group an ideology develops. To be a long-term member of that group you must adopt the ideology held by the majority of the group or at best leave, at worst, be expelled. Certain ideas and beliefs (yes, unfounded beliefs) must be adhered to, embraced, or you will be asked to depart.
Atheists claim to be open-minded, and perhaps that is the biggest problem… they are so afraid of being labeled closed-minded that they will embrace everything no matter how illogical or immoral as long as they can gain the approval of the group. They are adept at finding “logical reasons” for these illogical concepts. Just as fundamentalists have no tolerance for those who do not hold to the group ideology, neither do atheists and they will attempt to shame you, insult you, or threaten you in other ways, to make you conform.
There are a few atheists who have seen the hypocrisies of the group ideology and will converse with you with civility. They will not however confront the group they are in or attempt to make correctness for fear of losing the only thing offered by religion and these groups: socialization.
This is why I have resigned from all atheist groups and organizations of late. This, and the move of feminism into atheist groups has made such groups totally propagandistic. Just as many of the stances of atheist groups lack a foundation, feminism lacks any basis for existence whatsoever. I will not support any group or organization financially. I will not provide monies to ideological movements of this sort.