Here's the experiment as explained in the book:
You have a serious conversation with a deeply christian friend. Your friend is intelligent, well educated and knowledgeable. You agree to record the session. The topic is islam. During the session, you discuss that mohammed was a self-appointed prophet and that he claimed he talked to allah and the angels. He wrote a book that he claimed was infallible, and he flew from Jerusalem to heaven on a horse.
During the conversation, you agree that mohammed was probably delusional to think he could talk to god. You agree that the koran was clearly written by mohammed and not allah. It is ludicrous for him to claim that he is the last prophet and that all others are false. Neither you nor your friend can believe that he flew to heaven, let alone on a horse. It all sounds too crazy, and you both agree it is difficult to see how someone could believe such a religion. At the end of the conversation, you say that muslims did not choose their religion; they were born into it. Anyone who was exposed to both christianity and islam would see that christianity is the true religion.
Over the next few days, you transcribe the recording onto paper. Then you change all references to mohammed and make them jesus. Now the document reads something like this:
During the conversation, you both agree that jesus was probably delusional to think he could talk to jehovah. The bible was clearly written by men and not jehovah. You both agree it is ludicrous for jesus to claim that he is the last prophet and that all later ones are false. Neither of you can believe that he rose from the dead, nor flew to heaven. It all sounds too crazy, and it is difficult to see how someone could believe such a religion. At the end of the conversation, you both agree that christians did not choose their religion; they were born into it. Anyone who was exposed to both christianity and islam would see that islam is the true religion.
Now, tell your friend, "I made a transcript of our conversation about islam and would like to go over it with you." As you read it, watch her reaction. How does she respond to each statement? How soon does she get defensive? How quickly does she start making elaborate arguments that have no more factual basis than the first conversation? If you persist in this line of parallel reasoning, how long before she gets angry or breaks off the conversation? Could this conversation damage your friendship?
You can do this experiment with other prophetic religions. For example, substitute Joseph Smith for mormonism or moses for judaism.
Have you ever tried an experiment like this? Have you ever questioned your friend's religion? Did it damage your friendship? Or a family member? Does that person still talk to you? How far did your religious friend go to make excuses and explain away his religion in the face of reality?