The existence of God...

… God does not belong to the class of existing things: not that He has no existence, but that He is above all existing things, nay even above existence itself. For if all forms of knowledge have to do with what exists, assuredly that which is above knowledge must certainly be also above essence (υπερ ουσιαν); and, conversely, that which is above essence will also be above knowledge.
St. John Damascene