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 fields overlapping with religion, such as sociologists, philosophers, and anthropologists) 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 articulated within the phrase “That without which the subject would not be what it is.”
It was believed that without its essence religion would simply not exist. Seeking after religion’s sine qua non would thus constitute a rather daunting project, and in their attempts to discover this component theorists proposed numerous hypotheses based upon their interpretation of the data available to them. 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,” and Erwin R. Goodenough the “Mysterium tremendum.” Others were far more negative. For example, Sigmund Freud viewed religion’s essence as being wish fulfillment and illusion, Ludwig Feuerbach and Karl Marx as mere projection, and to David Friedrich Strauss wish-making aspiration. Despite the diverse views presented concerning religion’s essence these theorists all agreed that religion exhibited a reality. Obviously whether this reality is true or false would depend on the theorist.
It is clear that many of these theorists operated with and within the Kantian Paradigm that had already been established prior to them. There is a clear working with Kant’s tripartite categorical formula which proposed three a priori categories (thought, ethics, and aesthetics) believed to be the sine qua non of religion. Theorists after Kant worked with these categories by adding, modifying, or rejecting them. For example, Schleiermacher worked within Kant’s category of feelings and attempted to elaborate on its contents while Otto went beyond Kant to propose his unique category of the Holy.