For the record, I think Jesus probably had some kind of religious experience during/after his baptism, because duh. 

As to how would you prove this, basically you can’t prove anything, and it’s just a matter of what’s probable, given the information we possess.  We assume Jesus was baptized by John because early Christians would have preferred that not be the case, but we also assume the gospel writers dressed it up a bit to make it seem Jesus was still more important than John.

Mark’s account of the baptism doesn’t say anybody else heard God’s voice–only Jesus.  Let’s assume he told some version of that story.  How you could ever prove what somebody did or did not hear inside his own head–????? 

Paul shared a very sketchy version of his revelation on the road to Damascus (that mentions neither a road nor Damascus).  We don’t know he had any such revelation.  We just know that he refers to it rather obliquely, and that there was apparently some more involved account of it that circulated among Christians, and ended up in Acts.  But is it likely that Paul would have changed directions so abruptly without some kind of experience like that?  No.  Do we know of incidents in modern times where people changed their lives in response to a vision they either had or claimed to have?  Obviously.  Do we know the human mind is capable of such visions?  As much as we can know anything about our own minds. 

So is this a durable pattern in human existence?  Self-evidently.  Does the story of said vision necessarily correspond well to what the person in question really experienced? How the hell would I know?  Nobody will ever know.  The people Jesus theoretically told this story to wouldn’t know.  Jesus himself might not have clearly remembered what he experienced, might have embellished it over time. Almost certainly would have done so. 

If we had it in Jesus’ own handwriting (that we would somehow know was his) he had this experience, that would just be proof he said he had this experience.  Lincoln told people he had dreams some people today think are prophetic.  Do we know he had these dreams?  No.  I had a dream night before last where two friends of mine were working at jobs they don’t have in reality, and my dog was in it.  Do you know I had this dream? I could be making it up to prove a point.  Also, my memory of the dream is a bit fuzzy.  I should have probably written it down when I woke up, but I didn’t know I was going to be using it to prove a point.

There is good reason to believe Jesus had a religious experience in connection to his baptism by John.  We don’t know it for a fact.  We don’t even know for a fact he was baptized by John, but there is better reason to believe that. 

But for me, the primary thing proven here is that I’m not the only one Robert sometimes has issues with.  That is a fact. 

😉

The vision, as described by Mark, was internal, not witnessed by others at the time.  There is nothing hard to believe about this, in the context of someone who belonged to an apocalyptic Jewish cult.  The man described to us does in fact frequently behave as if he were in the grip of a religious vision, so no reason at all to think he wasn’t, or that many others have not been.

The question, as you say, is did Jesus actually relate such a story to his followers, and does Mark give us a reasonable approximation of what he related.  The former seems very likely, the latter much less so, transmission error being what it is.  And Mark was an adoptionist, so it’s a bit convenient that Jesus is having a vision that confirms his adoption by God.  How exactly Jesus perceived his relationship with God and his role in God’s plan must be considered ambiguous, because we have nothing directly from him, and we don’t know Mark’s sources.  Very unlikely to have been firsthand, and firsthand sources would not be 100% reliable.

So I guess it comes down to what you believe.  I believe the baptism couldn’t have survived as part of Christian belief if Jesus hadn’t repeatedly underscored its significance.  Otherwise, they’d have tried to bury it–certainly wouldn’t have appeared in all four surviving gospels (though John’s gospel tries to subvert and negate it–which only goes to show how widespread the knowledge of it was). 

If the baptism hadn’t mattered to Jesus, it wouldn’t have mattered to his followers, and it wouldn’t have mattered to early Christianity, and we wouldn’t be talking about it now.  Why did it matter so much?  Because, one might reason, it was a turning point of some kind for him.  Why would that be? 

Mark didn’t have Q, though.  Gospel of the Hebrews is believed to have been written later.  Wherever this tradition came from, it was very widespread.  Which would tend to substantiate that it was something widely known to Jesus’ original followers, which means they probably got it from him.  Jesus himself was the source of the original baptism story.  If it had just been gossip from John’s followers insisting on their master’s priority, they could have written it out (as John the Evangelist eventually did).  Jesus himself emphasized the tremendous importance of The Baptist.  And to this day, you can’t read the synoptics with clear eyes, and not see how crucial John was to Jesus’ development.  And without wishing you could know more.