The Rabbi was a Jewish teacher and leader who lived on Gammu when the Honored Matres conquered that world in their return from the Scattering. The Rabbis was a very orthodox and conservative Jewish leader, always leaning to the conservative side, when confronted with spiritual dilemmas facing human-kind.
Jews had always been persecuted since before the days of Old Earth thousands of years in the past. But in an underground alliance with the Bene Gesserit, the Jews had received secret sanctuary from the Sisterhood. The Rabbi, who was never named by his first or last name, was the leader of his people, who included the Jewish Bene Gesserit Rebecca, and one of the Rabbi' assistants, Isaac; on the planet formerly named Giedi Prime.
The Rabbi found himself with the rest of the Jews congregated on the no-ship in which he and his congregation were kept in the vessel on Chapterhouse near the Chapterhouse Keep. When Sheeana Brugh, Garimi, Stokes, and the other conservative Bene Gesserit sisters decided to leave the now-combined Matres/Gesserit organization, the New Sisterhood, the Jews were caught up in their fervor. The group escaped with the gholas of Miles Teg and Duncan Idaho, as the latter took the no-ship off of Chapterhouse, and into the Unknown. They were stranded crew along with the imprisoned futars, Sheeana's six sandworm that had been raised near the Keep, and the last Tleilaxu Master, Scytale. As the great Outside Enemy, Daniel and Marty, tried to ensnare the no-ship in their Tachyon net, the entire group founds themselves in another universe as Idaho folded space.