The same deterministic physics that says that the spot where an electron ends up in the double slit experiment is not only nondeterministic, but doesn't exist in principle until it's measured.
While fair to say, this is kind of pedantry looking past the actual point. So, to be clear what I'm saying:
Unless reasonably proven otherwise, quantum mechanics are not a magical land of souls and consciousness. It's a probabilistic space. Those probabilities obey things like wave interference, it's not total chaos. And with Bell's, any idea of a "grand plan" dictating outcomes is kinda moot, because there's no hidden variables that would imply any kind of long-term vision, or mechanism therefore, at play.
Then, when you scale up to classical physics, those probabilities amortize out to reasonably deterministic outcomes: The sun will continue burning for billions of years. Technically every QM interaction in the core could suddenly get unlucky and it instantly poofs out, but we don't seriously consider that a possibility. Just like technically virtual particles could spontaneously form a Mercedes in front of me with a title of ownership in my possession while solar rays bitflip the hard drives of government servers to register it for me.
In reality, if I drop a ball, it's going to fall and hit the ground. When something scares a human, they release hormones. Those hormones consistently affect mood and behavior. Pretty much all humans think like most other humans, and all other animals seem to think like their peers. Etc.
There has been no evidence of any kind to suggest that human brains are not deterministic (and plenty of examples of otherwise), despite the technicality that they have QM underlying the matter and signals. And the idea that the quadrillions of QM interactions each instant are working to some conscious goal, using secret physics that have never before been observed and fly in the face of what has, is voodoo magic nonsense that is not based on actual quantum physics in any way.
If you want to believe in religion, just do that! Don't try to misconstrue/miscommunicate actual science to justify it, just let it be magic. God or souls are no more justified by quantum mechanics than they are by just saying they alter reality in any other way. You're not getting away from them having a magic will-based being in an unmeasurable qualia-defined pocket dimension altering natural outcomes.
In fact, you've made it worse. Now every single person has an unbelievable supercomputer like no one has ever imagined hidden, doing the math needed to understand the exact quantum manipulations to enact their will on the brain physics of the person they are somehow bound to.
To put it another way: if the land of souls exist, it has something to do with QM.
As for the "no evidence brains are nondeterministic" we don't have much evidence about anything related to brains.
My point is that between the two extremes - the blind faith in whatever religions have come up with, and the wingless reptile way of thinking that's so prevalent in modern science - I'd rather choose the middle ground and stay open to various ideas unless they are proven wrong.
What? Again, quantum physics, as we have observed them, directly contradicts your first sentence. It does not have a ghost in the machine!
And how in the world does one argue we have no evidence of brains? They're studied constantly! And there's tons of animals we can poke and prod! We know what regions do what, we know tons of mechanisms of actions. We know what hormones do what, we have ideas on how concepts like "time tracking" are done. We observe brain damage changing the personalities of people. We can do surgeries with relative levels of success.
Like: you don't even need all that. Do you get sleepy at night? Do you get grouchy when you get sleepy? Wonderful: that's hormones. Direct, experienced evidence that your thoughts and behaviors and actions are driven by hormonal states!
