A brilliant, charismatic, and misguided Vulcan, in 2287, Sybok used Nimbus III as a springboard for a mission to locate Sha'Ka'Ree and God. Sybok was Spock's older, estranged half-brother.
For his whole life, Sybok believed that Vulcans should not bury their emotions, but rather embrace them. By practicing what he preached, Sybok became the galactic equivalent of a religous crusader and zealot. Sybok also possessed the ability to put others in touch with their "secret pains" in such away that made the individual no longer burdened and willing to follow Sybok anywhere.
They followed him to Paradise City, where Sybok managed to hijack the Enterprise-A and take Kirk and crew safely through the Great Barrier, where Sha'Ka'Ree supposedly existed. With Kirk, McCoy, and his brother in tow, Sybok took a shuttlecraft to the planet and encountered "God." Unfortunately, "God" was nothing more than a malevolent alien, imprisioned on the planet. Sybok, shamed by is arrogance, redeemed himself by confronting the alien, which allowed Kirk and company time to escape "God's wrath."
Sybok did not survive the confrontation, but he did leave Kirk and crew with a renewed sense of mortality and a more enlightened understanding of what "faith" is all about.