God in Himself can be known only by Himself. He is different from us, not as an archangel is different from us – by some finite distance of hierarchical complexity. No, He is infinitely removed from us when it comes to what He is. He is what He is. This distance is also called the transcendence of God. What may be known of God by creatures, and what can be understood by analogy is the immanence of God. His transcendence is the infinite gap between Creator and creature, in essence and nature. “He had a name written that no one knew except Himself.” (Revelation 19:12)
When God declares “I AM THAT I AM”, He is explaining to us, as best as human language can communicate, what He is.
Philosophy, and specifically that branch of philosophy known as metaphysics, studies the nature of reality. It breaks things down into their component parts, hoping to reach the final essence that makes up the substance of things – be they atoms, protons and electrons, quarks, or according to one branch of science, strings of energy. Similarly, metaphysics classifies things by relating them to things more general or specific: genus and species. A dining-room chair is a species of chairs, which is a species of seating apparatus, which is a species of devices, and so on. Eventually we reach an abstract, general genus of “created objects” or “things”. Philosophy does this kind of exercise to know what things are.
God is not a species of some more general category, “godlike beings”. Nor are there species of God: sub-gods, demigods, emanations of God. God cannot be abstracted or specialised into something else. He is what He is. Similarly, God cannot be broken down or divided up into more basic elements that will explain the nature or substance of God. As we have seen, God is simple, and not composed. Metaphysics can do its work on the created order. But when it comes to God, it will eventually bend back on itself into circularity. What is God? God is what God is. I AM THAT I AM.
God is not being deliberately evasive or coy, nor is He obfuscating, when He calls Himself I AM THAT I AM. God cannot explain what He is with reference to something He has made.