I will prove to you that God exists

[Alan V]

I'm a theist but I'm not 100% sure. How can anyone be certain how a universe that caused intelligent life to exist was caused inadvertently by mindless natural forces? Do we 'know' such forces alone could do that? Do we know they did do that? Where does your certainty come from?

That's an argumentum ad ignorantiam fallacy, it is fallacious to argue that anything is true because we lack an alternative explanation. It is also an objective fact the the universe and natural processes exist, but there is no objective evidence any deity or anything supernatural exists, or that they're even possible.

One possible explanation for the apparent biofriendliness of the universe is anthropogenic multiverse cosmology.  Another is quantum cosmology, which Stephen Hawking developed because he wasn't happy with the multiverse idea.  "We learn from quantum cosmology that biological evolution and cosmological evolution aren't fundamentally separate phenomena but two vastly different levels of one giant evolutionary tree." -- quoted from On the Origin of Time: Stephen Hawking's Final Theory (page 259) by his associate Thomas Hertog.  In ways I cannot personally understand, quantum entanglement can create a holographic universe in which biofriendliness is an evolved characteristic.  But this is really a hypothesis rather than a theory at this point.

Quantum cosmology does not require the complexity of a whole other spiritual level to reality which theists prefer.  Their own hypothesis is anything but economical, since as you pointed out it goes so far beyond anything observable. So again, theists prefer God-of-the-gaps arguments, regardless of how unlikely they are in the face of what we already know about the universe.