In this book, David Michael Grossberg offers a fresh and illuminating perspective on the three-thousand-year evolution of Jewish monotheism by narrating the history of 'God is one' as a religious slogan from the ancient to the modern world. Although 'God is one' has been called Judaism's primary testimony of faith, its meaning has been obscure and contentious from its earliest emergence. From the Bible's acclamatory 'the Lord is one' to Philo of Alexandria's highest Word just secondary to God; from the Talmud's rejection of 'two powers in heaven' to the philosophers' First Existent who is one beyond unity; from the Kabbalists' tenfold Godhead to Spinoza's one substance, this innovative history demonstrates the remarkable diversity encompassed by this deceptively simple Jewish statement of faith. Grossberg shows how this diversity is unified in a continuous striving for knowledge of God that has been at the heart of Judaism from its earliest beginnings.