The ideal of public reason, made prominent by John Rawls, has become a mainstay of discussions about the proper role of religious arguments in a politically liberal society. In particular, Rawls's theory of public reason requires citizens and public officials to refrain from appealing to comprehensive religious and philosophical doctrines in public deliberation on matters of basic justice and constitutional essentials. In this essay, we review the ways in which the public life of Martin Luther King, Jr. — with its frequent appeals to a comprehensive doctrine to justify disobedience to the law — represents a challenge to the ideal of public reason, and we consider several Rawlsian rejoinders. What is missing from the existing body of scholarship on public reason is a thorough analysis of King's philosophical and theological arguments, including the examples of legal injustice he offered in his celebrated “Letter from Birmingham Jail.” As we note, King's specific examples of unjust laws rely on a theological framework that bedevils the attempt to reconcile his Letter with the constructivist underpinnings of Rawls's theory of public reason. Indeed, Rawls is in something of a bind: either King's argument is not acceptable under the terms of public reason or public reason simply cannot limit contemporary public discourse in the way Rawls has in mind. We consider several possible Rawlsian arguments for the accommodation of King's theological rhetoric, but conclude that the Rawlsian idea of public reason remains deeply problematic.