First, keep it clean and no flaming.

Now, this is an area I don't get to discuss too much, as the aethist is usually quieter about their beliefs than the religiously inclined. But they may feel just as strongly as the others, just as passionate.

I'd like to hear some reasoning behind how one develops the belief that there is zero chance a metaphysical being could have had any part in the creation of the universe.

Let's set some guidelines

There being a metaphysical "prime mover" does not necessarily mean "life after death"

it may not be responsible for the creation of life

nor the creation of a heaven, or a hell.

it may not have any interest in humans

nor any awareness of humans

at it's lowest common denominator, a supreme being, a "prime mover" is simply a metaphysical entity that is responsible for creating something from nothing. Specifically, this universe where there was no universe to start with.

But that also doesn't mean that if there is a prime mover, that it is limited to the lowest common denominator. It could be more than that too.

As little as we know about everything, I don't see how anyone can say with certainty whether from a random quantum fluctuation or from a metaphysical being.

It may be possible we are not even asking the right questions?