Over the course of more than half a century, Rabbi Dr. Irving Greenberg has developed a distinctive theology of intra- and inter-group relations. Deeply influenced by his experiences in the Christian-Jewish dialogue movement, Greenberg's covenantal theology and image of God idea coalesce into what I refer to as Hybrid Judaism, a conceptualization that anticipated key aspects David Hollinger's notion of Postethnicity. As such, Greenberg's system of thought is mistakenly categorized (by himself, as well as others) as an expression of pluralism. The twentieth century arc of social theories of group life in America, from Melting Pot to Postethnicity by way of Cultural Pluralism, serves to highlight the fact that Greenberg is better located at the latter end of this arc (Postethnicity), rather than in the middle (Pluralism). Central to Greenberg's proto-postethnic theology is the recognition of the transformative power of encounter in an open society. Greenberg's ideas are themselves the product of such encounters. Understood fully, Hybrid Judaism has great relevance for American Jewish identity in the twenty-first century.