It should be, according to the grammar, but it's not. The common definition is that Agnostics aren't sure about God, but Atheists are CERTAIN there is no God. Religion in the broad sense is one's belief system, whether it's a structure of things one can prove, or beliefs beyond the physical. Atheism is a religion because it's a belief system that is certain of something, which structures your world view. Science is another religion--that the world is primarily physical, and structured in a certain way. Oddly then, it's agnostics that are not s religion, because they're just not sure: it's not a belief system. In the main, they're not saying one CAN'T know, which is a rigid belief, just that they personally don't know. Yet. So yes, the dictionary and I disagree with you.