Mormons say that once you experience the Spirit testify to you that something is true, it is pretty hard to convince yourself that you did not feel the Spirit. I felt what I used to call the Spirit many times, I now feel forced by the evidence to have to think differently about those powerful experiences. I now believe that those experiences are produced entirely by the biological brain.
When I was nine, I had an experience that I interpreted as being visited by an angel. I no longer interpret it that way. When I was 14 years old, my mother passed away. On the day of my marriage in the temple, I believed I felt her presence there. I now believe that that experience was entirely produced by my brain.
Experiences are always vulnerable to reinterpretation when we adopt a new perspective. So, the argument that "if you had only experienced what I have experienced, then you'd believe" does not have any validity for me.