ineffable

ineffable \in-EF-uh-buhl\, adjective:

1. Incapable of being expressed in words; unspeakable; unutterable; indescribable.
2. Not to be uttered; taboo.

. . .the tension inherent in human language when it attempts to relate the ineffable, see the invisible, understand the incomprehensible.
-- Jeffrey Burton Russell, A History of Heaven

Pope John Paul II notes that people are drawn to religion to answer the really big questions--for example, "What is the ultimate ineffable mystery which is the origin and destiny of our existence?"
-- William A. Sherden, The Fortune Sellers

One cannot blame them very much; explaining the ineffable is difficult.
-- Edward O. Wilson, "The Biological Basis of Morality", The Atlantic, April 1998

Ineffable is from Latin ineffabilis, from in-, "not" + effabilis, "utterable," from effari, "to utter," from ex-, "out" + fari, "to speak."