Without citing the bible, what marks the bible as the one book with God's message?

[Paleophyte]

The origin of the epistles:  Saul  of Tarsus was allegedly on his way to Damascus when he fell of his horse and hallucinated.
As far as I'm concerned, that's calling a spade a spade:  If a person claims to have hear god speak to him and to have had visions, he is deemed to be mentally well.   I suspect suffered from epilepsy or perhaps Schizophrenia.

Diagnoses across such a great length of time and befuddlement are fraught with peril. The first difficulty would be establishing the existence of the patient.

The simpler explanation is that Saul of Tarsus was a convert who wanted the head job. Having never met Jesus he simply faked an audience with the big guy in order to establish some cult cred. It's a pretty standard power struggle that you see played out in many modern religions.