Most people these days understand the meaning of the word 'God' as follows:
God is a supernatural being who knows everything, can do anything, and is perfectly good.
We can call belief in “God” according to this definition “classical theism.”
I want to propose a quite different way of defining “God”, as follows:
God is the thing that is known in true mystical experiences (if there are any).
We can in turn define a “mystical experience” as follows:
A mystical experience is an experience which, for the person having it, seems to directly reveal something which is:
unchanging and eternal
ineffable, i.e. its nature cannot be put into words
of great value, and such that the awareness of it is the supreme value of human life
Mystical experiences have existed in all cultures throughout history, and such experiences have inspired much of our greatest poetry, such as that of Wordsworth and T. S. Eliot. Whether there are any true mystical experiences, that is, whether there are any experiences which really do reveal something eternal and ineffable, is of course controversial. However, understood simply as psychological episodes that may or may not be delusions, the reality of mystical experiences themselves is beyond doubt.
Why define “God” in this way? Because I’m inclined to think the primary basis of religion throughout history has been the mystical experiences of various prophets and sages, such as Moses, Jesus and Buddha. Some of these prophets might have also (for whatever reason) taken God to have certain more specific characteristics, e.g. it seems that Moses and Jesus took God to be some kind of supernatural person. Still their primary way of fixing on the thing they called “God” was via their mystical experiences, and hence it is appropriate to capture what they meant by “God” in terms of these experiences, rather than in terms of some more specific definition.
My definition of God allows that our understanding of God might evolve over time. Primitive people tended to anthropomorphise the things they experienced, e.g. attributing personality to the moon and sun. We might see primitive religion likewise as anthropomorphising God, taking God to be some kind of supernatural being with the characteristics of a person. But even if this was a mistake, it doesn’t follow that God does not exist. When they gazed up in the sky, primitive people experienced real things, i.e. the sun, moon and stars; they just misunderstood their nature. Similarly, it could be that in his mystical experiences Moses experienced something real; he just misunderstood its nature. Yahweh might really exist, even if the Old Testament prophets were radically wrong in certain respects about what Yahweh is.
Thus, my definition of God has considerable flexibility. But it doesn’t make the existence of God entirely vacuous. If could turn out that all mystical experiences are delusions, in which case God does not exist. However, we cannot prove atheism simply by demonstrating the falsity of classical theism (e.g. through the problem of evil), or even by proving the non-existence of anything supernatural (e.g. by inductive argument that the physical world is a causally closed system). For it could be that:
Certain gifted individuals, from Moses to Wordsworth, really do have direct access to an eternal and ineffable reality,
The eternal and ineffable reality experienced by such individuals is nothing other than the essential nature of the material universe.