This article draws on archival sources to offer the first thoroughgoing account of how John Rawls moved from his undergraduate Christian ethics to the mature moral theory that undergirded A Theory of Justice. Identifying the liberal Protestant (rather than neo-orthodox) convictions at the heart of Rawls's senior thesis, it shows how he found an alternative postwar grounding for these views by applying Wittgenstein's later arguments about concepts, criteria, and inductive reasoning to ethics. The article places Rawls in the context of a whole community of mid-century American ethical theorists drawing upon Wittgenstein and trumpeting forms of moral “naturalism,” many of whom shared Rawls's Protestant heritage. It suggests that we can only make sense of the reliance of Rawls's moral theory on personal recognition, emotion, and a universal vision if we sideline the traditional dichotomies of the secularization debate and regard the postwar ethics of Rawls and his cohort as “post-Protestant.”