In his famous vision of the enthroned and exalted Lord, Isaiah is told to harden the hearts of his people, ‘lest they repent’ (Isa. 6.9-10). According to Mark’s Gospel, which cites this text, Jesus speaks in parables for the same reason. Interpreters of all generations have, not unnaturally, found the text difficult and disturbing. This study is concerned with the fascinating variety of early Jewish and Christian interpretations and modifications that have attempted to cope with the difficulty. At different stages in Judaism and Christianity the Isaianic vision of God is more clearly perceived than at other times. Isa. 6.9-10 is itself indeed shocking and disturbing, but this is not inappropriate in the context of the prophetic message.