How does that differ from the fellow who believes that the Man in the Sky tells him what to do? I knew a fellow who heard the Voice - the diagnosis was "madness" (a diagnosis made decades ago when psychology was still in the Dark Ages).
And how does that differ from the fellow who believes that the Man in the Sky tells him what to do? But this time, no voice - the answer just "comes to me."
If someone believes foolishness about technology, sports or politics we're free to enlighten them. Like a fellow who believed that he could better his odds of winning the lottery by choosing numbers with a random-number generator. So why should we not criticize "religious thought"? Humans believe wrongly in every field. They should always be open to correction. There's nothing sacred about religion.
It is impossible to respect other peoples' opinions - there are too many of them, they conflict and they can't all be right. But you _can_ respect the other people. I believe that is what etiquette is for. Good manners can save the day.
I mean just consider the comparison you just made. How many schizophrenics have you heard of who can even keep their lives together without medication? It's a very serious condition. Compare their lives to that of the average churchgoer. It's not even debatable which you'd prefer to have.
You may have the good sense to keep those thoughts to yourself but don't be surprised if you share that no one really thinks highly of you for it.