IIRC, in the novelization Sybok first hears "god" as a child while attempting to access his recently-deceased mother's katra in the Hall of Ancient Thought (or what-not...it's where Spock's katra would have ended up if his body hadn't been regenerated, under normal circumstances).
I don't think Sybok really set out to create a 'cult' except as a means to an end, though I'm sure he'd welcome anyone trying to find god the way he did, and he likely had some true believers among the many seemingly brainwashed people.
I don't really have a problem with the quest in and of itself...it's his methodology that's problematic. All things being equal he could have been an amazing psychotherapist, though it's unclear whether he was really helping people deal with their pain, or whether that was a short-term side-effect of swaying them to his side.
Of course, if we assume he's a zealot, rather than just a true seeker, and that his efforts to acquire a starship (a Starfleet vessel, not just any old run-of-the-mill ship) were because nothing else had the power to penetrate the Great Barrier, then it was essentially inevitable that he'd run afoul of the law if not morality in the process.