The article deals with a comparative analysis of the structure and content of religious experience, specifically the experience of “mysterium tremendum”, in the works of Semyon Frank and Rudolf Otto. It reveals the connection between the concepts of “mysterium tremendum”, “mystery”, and the “the utterly different” as used by the authors. The author attempts to identify perspectives in S.L. Frank’s works that reference the idea of the “sacred” in R. Otto’s work, and to articulate their significance for S.L. Frank’s ontology. The article considers recently introduced works by S.L. Frank, including the text “First Philosophy” and lecture notes on the “Philosophy of Religion”. It is argued that S.L. Frank describes the nature of the “Holy” and the structure of religious experience by drawing on the framework of religious experience from R. Otto’s work “The Sacred”. Like the “numinous”, the Holy is described by S.L. Frank as a mystery that evokes awe and is compared to R. Otto’s phenomenological category of “mysterium tremendum”. At the same time, the work “The Unknowable”, as the author calls it, is “an ontological introduction to the philosophy of religion”. The categories of R. Otto’s phenomenology of religion are given an ontological character, and they are integrated into the structural principle of reality developed by S.L. Frank – the principle of antinomic monodualism.