The Cult of Truth is a sect of magic users, whose magics are directed at defensive, tool and clerical spells, appearing in Ultima III. At that time, the Cult of Truth was already over 3000 years old. They wrote "The Ancient Liturgy of Truth", which is part of the game package. Therefore, their spells apply for the Clerical classes in the game, not the Wizards. It is also known that Alizarkanon is one of the prophets, addressing the reader of the book in a foreword.
Their strongest, and never copied, spell was "Anju Sermani", which allowed the cleric to return someone to life even if completely burnt to ashes. Considering the price that has to be paid for the spell, it is no wonder why it vanished into the darkness of time.
After Ultima III, the Cult of Truth vanished, most likely destroyed by the cataclysm.