To be fair, a metabolism is a biological system.
That said, atheism isn't 'system (of belief) without a god', but rather 'without the system (of belief) pertaining to god'. Which is why the definition of atheism is 'disbelief or lack of belief in god or gods'.
With disbelief being: 'inability or refusal to accept that something is true or real'.
We're unable to accept that god is real. It's not a conscious choice, but one stemming from the (lack) of credible evidence regarding the claim being made.
That said, atheism isn't 'system (of belief) without a god', but rather 'without the system (of belief) pertaining to god'. Which is why the definition of atheism is 'disbelief or lack of belief in god or gods'.
With disbelief being: 'inability or refusal to accept that something is true or real'.
We're unable to accept that god is real. It's not a conscious choice, but one stemming from the (lack) of credible evidence regarding the claim being made.