The modern period saw innovations in Europe that lead directly to contemporary ideas about “God is one,” this chapter’s subject. From an intellectual-historical perspective, modernity is in surprisingly large part a matter of by what methodology and by whose authority the truth or falsity of various types of propositions – moral, philosophical, scientific, or theological – ought be judged. The major trend was that other sources of authority, importantly critical reason, had primacy over tradition. Yet “God is one” remained a stubbornly persistent notion. Even as radical a thinker as Spinoza was committed to it. A more traditionalist modern Jewish perspective sees “God is one” as a “slogan of faith” with no content, not about God but about the religious practitioner’s intentions. Knowledge of God is impossible, but an intention to serve God, even if always beyond human ability to realize, is possible. This aspiration is expressed when declaring, “God is one.”