God either does or does not exist already so the “real” probability would be 0 or 1.
Not true. There are innumerable definitions of God. They range
- From Spinoza's
God = Universe
ie pantheism - ... through panentheism — Spinoza is true but Mainstream Christianity is also true
- Christian God
- Pastafarianism — explicitly set up to invalidate it's own 'God'
- Much much else not on this spectrum
So the real question about God is not whether He (she it) exists, but what do you mean when you use the word.
To use an analogy from logic: You can argue whether a formula is valid (always true), contradictory (always false) or conditionally true ie depending on the free variables.
But all this is itself conditional to the formula being well formed
To quote the physicist Pauli you can be right wrong and not even wrong.
Does God exist?
needs not Y/N but a P(auli) answer.
In any case, here's thean unassailable probability argument why believing in something God-ish trumps not believing
All but five of the 263 centenarians we interviewed belonged to some faith-based community. Denomination doesn’t seem to matter. Research shows that attending faith-based services four times per month will add 4-14 years of life expectancy.