In another life, Sybok was a glitzy televangelist wearing a fancy suit and talking a mile a minute. But this is the sci-fi, future world of Star Trek, so what we get is a unique Vulcan who has shed the love of his species for logic in favor of pure emotion, in the process setting out on a massive spiritual quest.
The Emo Vulcan
Oh, yeah, did we mention that this dude is Spock's half-brother? Because he totally is.
Before marrying Spock's human mother, his pops, Sarek, had a child with a "Vulcan princess," and that child was named Sybok. Not the best name, but we'll take it. According to Spock, Sybok's early potential was dashed when he ran afoul of Vulcan society:
SPOCK: He believed the key to self-knowledge was emotion, not logic. When he encouraged others to follow him, he was banished from Vulcan, never to return.
In other words, Sybok is too emo. Long ago, the Vulcan species made the active choice to abandon emotion in favor of logic, and since then, feelings have been strictly verboten in their society. In defiance of this, Sybok argued that emotions are the only true way to achieve "self-knowledge."
Off the Deep End
At some point, Sybok's beliefs took on religious implications. He gained the ability to identify individuals' "secret pain" and release them from it, thereby turning them into loyal followers. He also becomes obsessed with locating Sha Ka Ree, the Vulcan equivalent to heaven or Eden, which is located at the center of the galaxy.
Still, he's no fundamentalist. He sees all religions as equal:
SYBOK: Sha Ka Ree. The source. Heaven. Eden. Call it what you will. [...] Still every culture shares this common dream of a place from which creation sprang.
In other words, according to Sybok, all religions tell the truth, even if they might frame that truth differently. This is one of the more relatable aspects of his belief system.
What he doesn't mention initially, though, is that he got this idea from a vision. Makes him seem a little more wackadoodle, huh? What's more, it's eventually revealed that the "god" who sent this vision is, in fact, a galactic prisoner who wants Sybok to break him out. What Sybok thought was a religious crusade was nothing more than a long con.
In the end, however, Sybok proves his fundamental decency when he sacrifices his life in an attempt to take down the imposter. It's an important reminder that, though Sybok is as misguided as someone who prefers the Backstreet Boys to 'N Sync, he's driven by a sincere desire to do good and relieve others' suffering.