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Captain
The fastest growing religion is islam.
Only because the regions of the world with the highest birth rates are also those where Islam predominates.
Atheism is not comparable to any religion because it isn't one. It has no doctrine or dogma, it is simply the absence of any religious beliefs.
Sociologically I think Atheism can be considered as equivalent to a religion. And even today we're seeing the faint stirrings of a brand of intolerant non-faith hardly dissimilar from religion's past and present persecution of non-believers.