Does God exist?

I have a friend – a medical doctor – who firmly believes that God does not exist. Though she does not say it openly, she puts up a front that she is agnostic. She disagrees with me when I say that acceptability of any belief rests on its utility and not necessarily on its factual correctness. I have in one of my books (“How and why of Yoga and Meditation – Yoga Scientifically explained”) dealt in detail how devotional singing can be utilized to relieve one of mental stress. I have made it clear in that context that it works irrespective of the existence of God or otherwise. Just the belief is enough to make it work. I have also discussed some research findings in another of my book (“How does the mind work?”) where mere beliefs seem to bestow miraculous results. In either case, the factual correctness of the belief is immaterial.