The finding of the pronounced divine name Iao freely used in a LXX MS from Qumran "has left many scholars baffled" (Kristin de Troyer). This name is best known from gnostic and magical sources, but till now no one has provided a history of its background prior to its move into these genres, necessary for placing it in the LXX's textual tradition and understanding the chronology of the gradual disuse of God's name in non-mystical contexts. This book presents new evidence (especially onomastic and classical) for, and reveals problems with prior scholarship's positions on, the continued non-mystical pronunciation of the divine name among some within second temple period Judaism, and precisely when the name Iao moved into the mystical sphere. Key matters addressed include the divine name's forms in the LXX and NT. The work also contains the first English translation and commentary of Byzantine polymath John Lydus' chapter on the Jewish God from De mensibus 4.53.