Associate Justice Hugo Black is often considered one of the giants of twentieth-century American religion clause jurisprudence. Especially regarding the Establishment Clause, Black sought to leave his mark on precedent. Previous biographers and legal scholars have noted the influence of his own religious convictions on his legal reasoning. I extend this line of inquiry but argue that Black's decisions enshrine a more concrete, substantive view of religion and political life than has previously been acknowledged. By drawing primarily on archival research regarding Justice Black's reading, correspondence, and religious membership, I argue that we can best understand his religious thought as a species of political theology, one I term syncretic civic moralism. In brief, Justice Black viewed the ideal religion as one free of doctrinal claims and primarily supporting prosocial behavior and civic loyalty. After outlining the impact of his theology on his landmark opinions, I conclude by suggesting some of the consequences of Black's theo-political jurisprudence for contemporary American establishment debates.