8 But look at this which is said by Christ the Lord: “If you believe not that I am, you shall die in your sins.” What is this, “If you believe not that I am?” “I am” what? There is nothing added; and because He added nothing, He left much to be inferred. For He was expected to say what He was, and yet He said it not. What was He expected to say? Perhaps, “If you believe not that I am” Christ; “if you believe not that I am” the Son of God; “if you believe not that I am” the Word of the Father; “if you believe not that I am” the founder of the world; “if you believe not that I am” the former and re-former, the creator and re-creator, the maker and re-maker of man—“if you believe not that I am” this, “you shall die in your sins.”
There is much implied in His only saying “I am;” for so also had God said to Moses, “I am who am.” Who can adequately express what that am means? God by His angel sent His servant Moses to deliver His people out of Egypt (you have read and know what you now hear; but I recall it to your minds); He sent him trembling, self-excusing, but obedient. And while thus excusing himself, he said to God, whom he understood to be speaking in the person of the angel: If the people say to me, And who is the God that has sent you?
What shall I say to them? And the Lord answered him, “I am who am;” and added, “You shall say to the children of Israel, He who is has sent me to you.” There also He says not, I am God; or, I am the framer of the world; or, I am the creator of all things; or, I am the multiplier of the very people to be delivered: but only this, “I am who am;” and, “You shall say to the children of Israel, He who is.” He added not, Who is your God, who is the God of your fathers; but said only this: “He who is has sent me to you.”
Perhaps it was too much even for Moses himself, as it is too much for us also, and much more so for us, to understand the meaning of such words, “I am who am;” and, “He who is has sent me to you.” And supposing that Moses comprehended it, when would those to whom he was sent comprehend it? The Lord therefore put aside what man could not comprehend, and added what he could; for He said also besides, “I am the God of Abraham, and the God of Isaac, and the God of Jacob.” This you can comprehend; for “I am who am,” what mind can comprehend?
Source: Tractates on the Gospel of John (New Advent)