It looks like you put atheism and agnosticism in the same boat. There’s a certain belief, trust and conviction to lack of a deity in atheism. Not in agnosticism. That state of mind is orthogonal to belief is mostly true. On the other hand , there is also a motivation to explain away religious experience as physiological process, which explains the overlap in group membership.

> There’s a certain belief, trust and conviction to lack of a deity in atheism

Atheism means a lack of belief in a god. Just because many atheists go a step further or the word agnostic exists, that doesn't change the meaning.

I have yet to see an "official" source that says it's definitely belief of absence and not absence of belief. Agnosticism works just as well if you view it as subcategory of atheism.

My understanding is atheism is not believing in a deity (a - without, theism - belief in deity) and agnosticism is about not knowing (sometimes not knowing if it's possible to know). It gets messy because plenty of folks play games with the words like "clearly I don't believe the major, self-contradicting religions but maybe there is some deity out there somehow that had an influence beyond 'natural processes'" or that Einstein quote about calling the beauty of physics and the universe "god".

Similar to what you said about me, I think you're perhaps putting faith/religious dogma and spirituality/holding things sacred in the same boat. I think having awe or wonder or a feeling of being part of something bigger is again orthogonal to belief in a particular god. I don't even think religion is the dividing line here - religion and ritual exists all over without an overarching deity.

I liked the underlying idea to what you said about motivation to explain away religious experience as physiological process; I think there is something interesting there. I expect this is a result of what people already believe, not a cause, but I like the concept of how people take in new information and default to directing it to "knowable, let's figure it out" or some version of "unknowable".

tl;dr - not believing in a god seems separate from spirituality and religious experience. Theist and atheist are extremely high level (and one dimensional) labels and there is a LOT of diverse (and overlapping) belief and experience under each.