Jesus said to the Pharisees: “I am going away and you will look for me, but you will die in your sin. Where I am going you cannot come.” So the Jews said, “He is not going to kill himself, is he, because he said, ‘Where I am going you cannot come'?” He said to them, “You belong to what is below, I belong to what is above. You belong to this world, but I do not belong to this world. That is why I told you that you will die in your sins. For if you do not believe that I AM, you will die in your sins.” So they said to him, “Who are you?” Jesus said to them, “What I told you from the beginning. I have much to say about you in condemnation. But the one who sent me is true, and what I heard from him I tell the world.” They did not realize that he was speaking to them of the Father. So Jesus said to them, “When you lift up the Son of Man, then you will realize that I AM, and that I do nothing on my own, but I say only what the Father taught me. The one who sent me is with me. He has not left me alone, because I always do what is pleasing to him.” Because he spoke this way, many came to believe in him.
Our Lord expresses His meaning from today's Gospel in the words, "You are of this world," that is, you are sinners. All of us are born in sin; all have added by our actions to the sin in which we were born. The misery of the Jews then was, not that they had sin, but that they would die in their sin. Amongst the multitude who heard our Lord, there were some who were about to believe; but this most severe sentence had gone forth against all: "You shall die in your sin;" to destroy all hope even in those who should hereafter believe. So His next words recall the would be believers to hope: "For if you believe not that I AM, you shall die in your sin": therefore if you believe that I AM, you shall not die in your sin.
Jesus saying, "If you believe not that I AM," proves a great deal. For it was this way that God spoke to Moses, "I AM WHO I AM." (Ex 3:14) We are to understand it in this way. Anything, if it can change, cannot be said to truly exist. Analyze the idea of change, and you will find a past and future; contemplate God, and you will find an eternal present without the possibility of a past. So then, "If you do not believe that I AM," means in fact, "If you do not believe that I am God"; this is the condition, on which we shall not die in our sins. Thanks be to God that He says, "If you do not believe," not, "If you do not understand"; for who could understand "I AM WHO I AM"?