The fact of religious diversity provides problems for a variety of approaches to the study of religion. In the philosophical study of religion, there are at least four main areas in which issues are raised by the fact of religious diversity: the epistemology of religious belief, the production of theories of religion, reflection on concepts of God and the Ultimate, and discussion of the relationship between religion and the human good. For the purposes of this article, J. L. Schellenberg's definition of religion in terms of “ultimism” is considered. There are at least four responses to the epistemological problems arising out of religious diversity that are worth discussing: apologetic investment, atheism, agnosticism, and anti-evidentialism. The epistemological reflections on religious diversity are thus vital to the interpretation of religion. There are three main types of theory of religion that arise from or bear upon the fact of religious diversity: naturalism, confessionalism, and pluralism.