But the divine mystery is inherent in the divine, a part of the nature of God, and can never disappear. And this means that it is still a mystery even to the mystic who has directly experienced it, nay, even to God Himself. That is why it is ineffable. The mystery and the ineffability of God are one and the same thing.
Time and Eternity - An Essay on the Philosophy of Religion (1952), p. 37