Is atheism a belief?

[Bucky Ball]

There is no such thing as "mind". What there are, are brains and brain processes, and they exist, because they promote evolutionary survival. 
Why anything exists at all is a Physics problem. Not a god problem. Same old bad habit at work.
Without healthy functioning brains, in every single case, there is not one instance of "mind". 
The fact you don't have an answer to something never justifies jumping to "well maybe god done it", whether you call that an "unknown in a box" or an "entity".
A philosophical entity would have to itself exist, therefore to say that one "allows for existence'' is not only meaningless,
but impossible, as it would have had to create the Reality it requires itself to exist, (and that's one of the insurmountable problems that makes ANY god incoherent). Existence is not nothing.

I'm pretty sure there IS a thing such as mind, since I'm capable of subjective experience.

As for brains and minds-- what, exactly, are your criteria for determining what physical systems do or don't have "mind"?  If I bring in a collection of material into Bucky Ball's laboratory, how will you determine whether it does or doesn't constitute an instance of "mind"?

Nope. Your subjective experiences are your brain interpreting what you experience. Sensory input, referenced to learned memory. No mystery there. 
I see you actually know nothing about neuro-science. The concept of "mind" is unnecessary. 

YOU are the one claiming you know there are "minds". You get to provide the evidence for them. You can't even define the word. 
You have NOT ONE instance of a mind at work, in the absence of a healthy brain, AND we have countless examples of humans with healthy functioning brains, (what I assume you would call a mind) which, when injured or diseased, stop working and poof ... no more "mind". 
"Mind" is a long outdated word and concept, used by people with no knowledge of neuro-science.
Neurology and neuro-science knows what parts of the brain are responsible for its various functions. It's entirely physical. No woo-woo "mind" needed.

I have no criteria for a "mind". The concept has no meaning. A "mind" might be "the things that brains do".
There is not one instance of what USED to be called a mind, functioning in the absence of a healthy brain, and you cannot come up with one.