Mystery, here, is not a secret, an unsolved question, a plot or an enigma (works of fiction). In philosophy, “mystery” describes a notion beyond ordinary human comprehension, requiring a deeper, intuitive approach.
In a spiritual or religious context, “mystery” refers to a divine truth or reality. Once accessed through direct intuition, it is by no means incomprehensible, but relatively inexpressible, difficult to communicate to another person who has not achieved the same intuition, beyond the strictly conceivable: the intelligible.