Atheism is a religion.

[Mister Agenda]

Atheism in certain forms can be viewed as religion as can many other ideas per the definition "a pursuit or interest to which someone ascribes supreme importance."

A religion could be atheistic, just as most religions are theistic. A particular religion is not a 'form' of atheism or theism. They are opinions on a single topic and either or neither may be a feature of a relgion or philosophy.

And using the definition you cite is equivocation when real religions are being discussed. It's like using the meaning of 'light' as 'composed of photons in a visible frequency' in a conversation about feathers and balloons. Yes, photons don't weigh very much, but that's not really the point of the conversation.

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Atheism in certain forms can be viewed as religion as can many other ideas per the definition "a pursuit or interest to which someone ascribes supreme importance."

I doubt that for many atheists, atheism is a "pursuit or interest" let alone one of "supreme interest"--rather fewer as a % than say chess enthusiasts or liberals or tea party conservatives. I'm enjoying the forum, but "supreme interest?" Hardly.

This. It's an opinion on one topic. It's hardly the most important thing to me, or to know about me. In fact, it's more important that people be aware that it's not that important; and that's not supremely important either.

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Whatever it is that forms the basis of their so-called church must be something other than atheism.
My guess is anti-theism. Attaching the name church to it is a bit odd but not equal to religion in the sense anti-theism is typically concerned with. Add a value system that includes political action and unfettered devotion to unsubstantiated claims and now you're in religion's domain.

I would guess it's some desire for community with like-minded folks. Nothing wrong with that. It doesn't make it church. Religions don't own the idea of community gatherings.