I apologise if it's come up before, but Agnosticism is not a valid religious belief: it is an epistemological position of the availability of conclusive evidence regarding the existence of a deity/deities.
As such, one can be an agnostic theist or an agnostic atheist - or simply a theist or an atheist. Theism is the explicit acceptance of the proposition "There exists a deity or deities." If one is unsure or undecided, then they are not a theist - that is, they are an atheist.
In light of this, you'll generally find that most people who refer to themselves as 'Agnostic' when questioned about their religious beliefs are in fact Agnostic Atheists.