I grew to understand the numinous and its difference from the rational in Luther's De Servo Arbitrio long before I identified it in the 'qadosh' of the Old Testament
Otto credits Luther's De Servo Arbitrio as the primary experiential source from which he derived his concept of the numinous, positioning Luther as the phenomenological origin of his entire analytical framework.
, The Idea of the Holy: An Inquiry into the Non-Rational Factor in the Idea of the Divine and Its Relation to the Rational, 1917thesis