In recent months I've often had cause to consider what it would take for me to believe in God. And each time I consider it, I come closer to the notion that there probably isn't anything that would convince me that God exists.
To be clear on this question, I should explain what I understand by the two particular terms, "God" and "exists".
"God" — for this purpose — must necessarily be defined as the God of Abraham, or something very like it, by which I mean it must have the characteristics usually attributed to the supreme creator of the Universe: omnipotence, omniscience, omnibenevolence, omnipresence, and having an existence somewhere and somewhen at least partially outside of what we understand as time and space. I don't include such woolly definitions as "God is energy" or "God is love", as these are neither specific nor useful.
"Exists" — for this and indeed most purposes — I take to mean having some causal relationship with at least some aspects of physical reality. For something to exist it must be perceivable (preferably measurable) in the same realm of existence in which we, the beings doing the perceiving, perceive it. And it must be physical reality, not some notional concept that "exists" only as an idea in a brain — such existence would be better termed "imagination". (I can imagine lots of things that don't exist in physical reality. The fact that I can imagine them does not confer on them any physical existence other than patterns in my brain.)
That's really as far as I need go. The notion of God described above is clearly incoherent. It's a concept that cannot exist in, or affect, the physical realm in which we reside.
Take omnipotence: can God create a rock so heavy he can't lift it? Can he create a square circle? I've often heard theists maintain that God can do anything that's consistent with the "laws of logic" — which those two actions are not. Fair enough, but then those same theists claim that their God created the laws, so all I need to do is reframe the question: can God create laws which even he can't break? There's no satisfactory answer to this that's consistent with the idea of an omnipotent God, so he fails the coherence test at the very first hurdle.
But never mind that, let's see how he stacks up in omniscience. This is the idea that God knows everything there is to know. All knowable knowledge resides in the mind of God. But that knowledge, to be complete, must also include the knowledge of the knowledge in the mind of God, which if it is itself to be complete, must also include the knowledge of the knowledge of the knowledge in the mind of God, and so proceed ad infinitum. We have here an infinite regress, rather like a subroutine which calls itself, and once more we're well into the realm of incoherence.
Omnibenevolence and omnipresence need hardly get a look-in — they are obviously incoherent. Omnibenevolence is shown to be nonsense by the fact that theologians twist themselves into knots making excuses (theodicies) for a God that (if it exists) clearly isn't all-good. Omnipresence is another nonsensical property, probably invented to reinforce the idea that God watches everything you do. (It's also a superfluous property — what need of omnipresence has an omniscient God?)
It's clear to me, therefore, that God has been defined out of existence. The only way these properties could be true characteristics of a being, would be if that being was entirely imaginary. Such a being would be a supernatural fantasy — it would not, could not, exist. My definition of "existence" above is necessarily restricted to the natural world. Anything supernatural, therefore, cannot exist.