Considering religion has a particular set of beliefs to the cause and origin of our universe, I don't find it valid. Unless Batman made the universe. However, to me, it seems like they are more of a community, than a religion.
Religion:
a set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs.
Source:
http://dictionary.reference.com/browse/religion
So, if Batman isn't a valid religion, then it can't be too obscure to be a religion, because it isn't one if it is lacking this fundamental belief of all religions.
Lastly, I have an adverse feeling against any human who believes they can provide "justice."