Do you ever doubt your atheism?

[Mudhammam]

It's just an idiom. So, not so much a theory as the sum total of what we do not know that has those attributes?

Yes, but an idiom that has historical significance as philosophers have long referred to this simple sum total (simple seems to be what we're striving at) as "God," though perhaps a name change is justified given the attributes superstitious minds have unfortunately heaped upon this incomprehensible source.

I'm not sure why we would conceive of that as a substance or even something that could can be encapsulated by a singular title, why it would be some singular -whatever- instead of many different things (especially in the case of necessity),

That's a fair point; it could consist of multiple necessary parts.

why it would be separate- or "free" (what does that even mean?),

Free as in not bound by the laws of causality--which we can only deduce as a principle that applies to finite, empirical objects, because otherwise it would simply be another part of the series and must then itself have a cause--I think QM has shown that this is in fact the case with certain quantum particles coming into existence seemingly without cause, free in the correct sense.

or unconditioned- or, why we would conceive of it at all. How does one go about describing something after claiming that it is "infinitely incomprehensible"?

It's infinitely incomprehensible in this way: If we examine "the logic of logic," we find at bottom certain paradoxes, for example, how a thing might be necessary and unconditioned, upon which all contingent and finite things depend upon, because otherwise nothing could have initiated the series; how space would seem to be infinitely divisible and yet finitely traversable between two given points, etc. While reason can lead us to concepts such as these, and in fact depends on them, it doesn't help us to comprehend them--whether it's a something that is non-spatial and non-temporal--or has existed for all eternity. I agree that it may appear

wholly and entirely entirely illogical,

But that just might be the essence or sum-total we can arrive at being incapable of conceiving anything outside of our experience (aided by the light of reason, which can only understand objects in experience, but by doing so depends on a logic that itself depends on incomprehensible categories of existence).

How did you arrive at such a place?

Fucking Immanuel Kant.