This essay challenges the view that John Rawls's recently published undergraduate thesis A Brief Inquiry into the Meaning of Sin and Faith provides little help in understanding his mature work. Two crucial strands of Rawls's Theory of Justice are its critique of teleology and its claims about our moral nature and its expression. These strands are brought together in a set of arguments late in Theory which are important but have attracted little sustained attention. I argue that the target of Rawls's undergraduate thesis is a form of Christianity which rests on assumptions Rawls later came to think were fundamental to teleological views, and that the thesis defends an alternative form of religiosity that anticipates what Rawls says in Theory about the expression of our nature. Those sections of Theory also provide resources Rawls could have used to respond to a number of prominent and recurrent criticisms of his account of moral motivation. Seeing the continuities between Brief Inquiry and Theory of Justice shows how long Rawls wrestled with problems he took up in the neglected sections of Theory and thereby shows their importance to Rawls's thought.