Inner experience is a poor reason to believe in God

I have no doubt that in cultures with strong religious traditions, many people believe in God without the benefit of any inner mystical experience. They believe what people close to them who they trust believe.

Another reason to believe is an inner experience. Suddenly a person feels God's presence. Or they feel a connection to the entire universe. It seems to be a very common feature of the stories of saints. These people are moved by the inner experience to go beyond what those around them believe.

Today, in more liberal circles, many people are atheists and if we follow the lead of trusted people it is definitely a live option. Among UUs, I sense that very few people believe because they have read the Bible (or the holy book of some other religion) and agreed with it intellectually. The more common reason to believe in God is some inner experience. This could occur while daydreaming or on the margins of sleep. It could occur during meditation. It could occur while reading a holy book.

In a sense, everything we experience or think we know is a perception. But some perceptions are more reliable than others. One criterion of clear perceptions is that everyone in the same situation perceives the same thing. This is the foundation of science. This liquid is a certain color. Looking at a life form, a given person sees the same body shape and characteristics. People can sketch it or photograph it, and everyone else who looks at it sees the same thing. You look at the gauge of an instrument and if it reads 97.3, everyone else who looks also sees 97.3. In more recent times, what you observe is often the output of a computer program. Other people can independently write a program designed to do the same thing, give it the same input, and see the same output.

Mystical experiences do not have this uniformity. People experience different things. Some have vivid experiences of being abducted by aliens. Society recognizes an entire class of such inner experiences under the heading of hallucinations, most often associated with schizophrenia or dementia.

There are also perceptions where almost everyone experiences the same thing but independent measurements of the scientific sort reveal them to be incorrect. Optical illusions are a classic case. In the 1970s I knew of the Necker cube illusion, the Muller-Lyer illusion, and a few others. Searching today, I found this large collection on the web, composed mostly of illusions I had never seen before.

Beyond perceptions, there are also inferences and beliefs. If I have a bible, it is "Thinking, Fast and Slow" by Daniel Kahneman. In it he lays out in detail the various ways that most people will think things that simply aren't true, with everyone tending to make the same mistakes. Mathematics, logic, and other observations of the repeatable, scientific kind give us in contrast the right answer. Wikipedia has an impressive (or depressing, perhaps) list of cognitive biases.

So even if everyone had the same mystical experiences, it would not be good evidence of the existence of God.

Inner experiences are very interesting in their own right. We live our lives in our own heads, and the part that reads scientific gauges, changes diapers, buys low and sells high, and generally gets things done is only one part. Various schools of meditation do achieve interesting inner experience that those who have not trained in their technique don't have. It's just that it all goes on within a single head. Or when information travels between heads it does so by through the mundane modalities of speech and writing.

An omnipotent God surely could give us experiences of the sort that scientific observation is based on. He could inscribe some particular message on prominent surfaces throughout the world. He could make us all hear the same message in our heads. We would independently write it down and discover that we all heard the same thing at the same time. We are so used to the fact that God never does this that we can lose track of how he surely could. One explanation for why he doesn't do this is that it doesn't suit his purposes. The simpler explanation is that he doesn't exist.

Some theists have argued that there is a God-shaped hole in each of us. To the extent this is true, the simpler explanation is that it is a "belief-in-God shaped hole". It could join the list of cognitive biases.

Mystical experiences are far from universal. They have a great deal of variability. It is hard to draw any clear distinction between them and hallucinations or alien-abduction experiences. Our inner experience is unreliable in a great many ways documented by science, typically in ways that are the same across people. Inner mystical experience is a poor reason to believe in God.