Perhaps what concerned earliest Enlightenment writers was the sine qua non of religion, the term used to denote a first principle or essence constituting the very fundamental core of religion.
Scholars of religion and other prominent theorists (not always necessarily religious scholars but academics active in other fields overlapping with religion, such as sociologists, philosophers, and anthropologists etc.) have attempted to apply methodological attempts to isolate this first principle. They wished to distinguish the core elements of religion understood to be absolutely fundamental to it. This fundamental component was best captured in the phrase “That without which the subject would not be what it is.” Simply, if this essence could be determined the idea would be that without the essence in question religion would simply not exist. Seeking after religion’s sine qua non would thus constitute a rather daunting project, and in their attempt to discover this component theorists have posited hypotheses based on on their interpretation of data. Numerous theorists constitute this effort. Rudolf Otto, proposed the idea of the Holy or the “numinous,” Friedrich Schleiermacher the notion of the “feeling of absolute dependence,” Anders Nygren the “eternal,” Erwin R. Goodenough the “Mysterium tremendum,” to name a few. Other theorists from across the theological spectrum, some of whom we will interact with here, too have proposed their own ideas. It is clear that many of these theorists operated with and within the Kantian Paradigm already established prior to themselves. We see a clear working with Kant’s tripartite categorical formula which proposed three a priori categories (such as thought, ethics, and aesthetics) believed to be the sine qua non of religion. Later theorists worked with these categories either adding to, modifying, or rejecting them. Schleiermacher worked within the category of feelings and attempted to elaborate on its contents, Otto proposed a fourth category of the Holy, and so on. Some theorists who were skeptical of or anti-religion, despite having an interest in the subject, made little attempt to conceal their ideas that religion at its essence was false and did not correspond with the way the world truly was.