Nearly every mystic reports it: the thing itself cannot be put into words. william-james listed ineffability as one of the four marks of mystical experience in The Varieties of Religious Experience.
Why? Language works by distinguishing one thing from another. What the mystics describe is usually before or beneath such distinctions — or all of them at once. Pointed at in poetry, suggested by paradox, the direct taste remains untransferable.
This is not an embarrassment for the traditions; it is a feature they insist on. The Tao Te Ching‘s first line declares it. The Upanishads teach by neti-neti. Zen‘s koans work by wrecking the conceptual grip.