When God announces that His name is I AM THAT I AM, He is communicating much of His nature and essence. Besides teaching us that His existence is grounded in Himself, the term I AM THAT I AM is about as concise a way of explaining the truth that God is simple.
The simplicity of God does not mean that God is easily understood or without infinite depth. Simplicity refers to the fact that God is not a being who is made up of various attributes, features or parts. God does not “have” or “contain” things that taken together, constitute God. All that is God is entirely God.
I AM THAT I AM could be said slightly differently. “I AM ALL THAT I AM”. “I AM WHAT I AM”. Nothing in God is only partly God. Nothing is merely an organ within God. Everything that is God is God in its fullness.
Consider a slightly gruesome illustration. Were you to be dissected into parts, those parts, taken together, would constitute you. We could keep dissecting and dividing till we reached the cellular level, and eventually the molecular, atomic and sub-atomic levels. These bits of you, when combined, form the being that is you. But no one would consider a singular atom of your body to be you in your fullness. You are the sum of your parts. You may be more than that, but you cannot be simpler than that.
God is not composed of atoms, or of whatever heavenly bodies are composed of. There are no divisible bits of God. That’s true not only of His substance, but also of His nature. God is not a composition of love, justice, mercy, holiness, omniscience, omnipotence, and other attributes. If that were true, the person of God would be a separate being from His attributes, and could only exist if these attributes combined to make Him, rather as we think of ourselves having or possessing “personalities”. Humans are composed of body, soul and spirit, or mind and affections, of memory and personality. If one or more of these is missing, a human is less himself, less real, less alive. Such a thing is not even logically conceivable for God. He cannot be divided. He has no parts. He is all that He is. Strictly speaking, He does not “have” attributes. He is His attributes.
A composite being must be an inferior being, for a being made up of parts is dependent upon those parts for life and existence. Further, if God were made up of parts, each of those parts would have some other origin, which would deny the eternality and self-existence of God.
Similarly, God experiences no tension between His attributes, because they are not differing parts of His being. His love does not wrestle against His justice. His justice is loving, and His love is just. He is justice and He is love; He does not merely have love and justice. It is all God. He is what He is. He is all He is. All that He is and does, is Him.
Even when thinking of the Triune God, we are delivered from tritheism or partialism by the truth of simplicity. God is triune; so all that can be said of God is triune. The Father, Son, and Spirit are fully God, because everything true of God is God in His fullness. Only the Persons stand as differentiated from each other, but each of these is still indwelt by the others and partaking fully of the divine essence. All that is God is as all that is God. He is all He is. I AM That I AM.
Simplicity, when rightly understood, does not make God immobile, distant, or almost impersonal. Instead, we have a God who is never merely “potentially Himself”, with passive, dormant attributes. He is always all of Himself: pure act. All that is God is present in all He is and does. To know God is engage with the fullest, purest life and being. All of Him in all moments. I AM THAT I AM.