I don't think I mentioned spiritual ideas. I don't disagree with that at all. Spirituality can exist outside the confines of religion.
But to state that you're an Atheist is to state that there is no god or gods, that religion is wrong. The atheist ideology (if you can call it that) exists only in the negative of religion. That is what I mean by 'oppose'.
You either believe there is a god/gods (Theism), disbelieve in that (Atheism) or you simply don't know and can't decide (Agnostism).
Disbelieving is innately oppositional, even if you as an individual is apathetic towards religion.
Still wrong. You can be agnostic within atheism. Atheism also does not require a positive claim that there is no god(s). A lack of a claim will suffice. "I don't know for sure that no god(s) exist, but for the time being I'm unconvinced and will live my life accordingly."