Other than personal experience, there isn't any. On the other hand there is no proof that God doesn't exist either. By logic we can establish that the monotheistic concept is so full of contradictions that it would be impossible for such a being to exist, but this is not the same as positive proof. The hardest problem is the problem of evil. If God made everything, which Scriptures attest, then God invented evil. This cancels any claims he has to being ethical (the ethical monotheism of Judaism). If God cannot get rid of evil, then Satan is either of equal or greater power. If of equal power, then the theology is really one of dualism (Zoroastrianism and Christianity). If of greater power, this doesn't say much for the omnipotence of God. Very intelligent and educated men have wrestled with these dilemmas, none of them successfully (under the banner of monotheism). Even so, this does not mean that there is nothing sacred. I call the laughter of a three year old sacred. I call the principle of Beauty sacred, as well as the principle of Being.