- Criticism of religion (e.g., explaining what is wrong with religion, why it is irrational, how it is detrimental to human progress)
- Promotion of atheism and/or secularism (e.g., encouraging others to make the break from religious belief and attempting to facilitate this process by providing a range of supports, working to strengthen the separation of church and state, attempting to improve public opinions of atheists)
If I created a group for atheist horror fans (which would be an odd thing to do), it would be silly to think that I'd attract atheists to the group who were not horror fans. Why would any atheist who was not a horror fan have any desire to be part of my group? They wouldn't, and so this would be a limiting factor on the size of my group. If I created a group for atheist liberals, I probably wouldn't have to worry much about growth potential. But because it is tough to imagine many conservative atheists wanting to join such a group, I'd have to be content with limited viewpoint diversity. Some conservative atheists might check out such a group, but few would probably want to stick around as active members. And even if they did, I'd wonder how they would be treated by the majority.
Creating a group for people who are atheists and who also share another interest or socio-political ideology is not necessarily a bad thing. I see nothing wrong, for example, with a group trying to attract atheists who are liberal, share humanistic values, and have a strong commitment to social justice activism. If they are up-front about their values and aim to attract members who share them, more power to them. The challenge for such a group is likely to involve the negative effects of limited viewpoint diversity, as well as how to prevent the imposition of ideological litmus tests, infighting, and periodic purges from harming the cohesiveness, functioning, and growth of the group.
The fact that atheism itself is quite limited in scope should not prevent atheists from coming together around any shared interest or set of views. The challenge when they do is how best to balance the common desire to reach more people, expand their size, and maintain viewpoint diversity with the equally common desire to maintain the mission of their group and minimize what they perceive as unwelcome trouble-making.