I have yet to be convinced that even the most fervent and militant atheists constitute a religion. A religion is a lot more than merely having one common point of agreement. Religions, generally, have a whole series of concepts, claims, myths, in other words a theology of some kind. Now I will grant that atheists who believe that science supports their view may come close (i.e. scientism), and certainly the way atheism was applied in the Communist states of the 20th century, as part of a larger package of ideological claims, certainly has come close to looking like a religion. I’m still uncomfortable with calling the more extreme forms of Marxist atheism as a religion, though I probably would concede the point if pressed.
But even a humanist organization more resembles, in my view, a political or ideological movement. But is that a religion? Is there are a creed, a common set of rituals, or anything that we normally associate with religion? Generally even the Eastern philosophies, which sometimes get lumped in with religions (i.e. Taoism), are, on their own, still philosophies. They are usually bundled up with myths and folk religion of the areas they evolved or adopted (for instance, in China, one finds a mix of philosophical world views, ancestor worship, animism and the like).
For myself, I can’t even call my lack of belief either scientific (it isn’t), nor do I see it as a spiritual view (being in awe of nature doesn’t seem sufficient to call it a faith system). In fact, my atheism is sufficiently weak that the only reason I even call myself an atheist is not because I don’t see how God isn’t possible, but rather I have yet to be convinced that God is a necessary part of the explanation. I don’t hold to Aristotle’s views, nor of the views of the Late Classical and Medieval Judeao-Christian philosophers that built on Aristotle. The most I’ll concede is that my atheism has a certain aesthetic appeal. I believe it is rational, but I do not confuse aesthetics and reason with Truth-with-a-capital-T, nor do I imagine that it can be demonstrated by empirical methods.