“Who knows only one religion knows no religion,” so Müller’s familiar dictum maintains, and the learned consensus generally concurs. To understand a given religion, most people agree, requires comparison and contrast with some other(s). Only then do we gain perspective, a sense of proportion and balance, an account of the choices a religion makes: for example, why one makes choices the contrasting religion may well reject. To compare, moreover, requires seeing two things as whole and complete, then brought into juxtaposition and relationship. And the very exercise of seeing a religious system in its entirety, carried out with contrast in view, requires making judgments about structure and proportion that comparison with another whole makes possible. So we study religion in comparison or not at all. And we compare religions because we seek a perspective on each and on all of them all together; that is, for the same reason that natural historians compare and contrast beetles, to see how they are alike and how they are distinct and to find out what difference that distinction makes—and to explain it all. But in the study of religion we are not yet able to explain very much. Indeed, we discern little agreement on just what, at this time, we ought to want to explain. So for the moment we compare religions that sustain comparison in order better to understand each one.
But what does it mean to know more than a single religion, and (more to the point) what does it take to draw into juxtaposition, for purposes of comparison and contrast, two or more religions? To know a given religion means to form a conception of the whole of that religion, meaning to grasp how it is cogent and proportionate, what matters and what does not, its architectonics and its hermeneutics and animating logic. All of this may be expressed in one word, “rationality,” meaning the cogency that imparts structure, the logic that propels the system. So to know a given religion means to grasp its principles of self-evidence and rationality, its category formation, its modes of thought and inquiry—to see the whole all at once