I think you and I subscribe by rather different definitions of atheist and agnostic.

Gnosis means knowledge, specifically of a spiritual kind, agnosis thus is a lack thereof. An agnostic would thus be someone that doesn't possess any hard knowledge of a spiritual kind. The way I see it, everyone is agnostic. If someone isn't agnostic then they surely have proof otherwise.

A theist is someone that subscribes by a faith, specifically with the belief of at least one deity (from the Greek word "theos"). It could be Abrahamic, Shintou, Hindu, Asatro, you name it. If there's a god, it's a theistic belief. So an atheist is just someone that doesn't subscribe to such a faith, though I recognise that it's used as "not having any religious conviction." Which to be fair, I use it as too.

I don't see spirituality and atheism as being incompatible though. You might be atheist and practise wicca, or I suppose even Buddhism.

To me personally though, when I say that I'm an atheist it means that I just don't really care. Religion isn't a part of my daily life. There's no belief in the existence or non-existence of any sort of deity, there's no thought on the matter whatsoever.