Just as Moses lifted up the serpent in the desert, so must the Son of Man be lifted up. The serpent was a sign of contradiction. The people were being killed by serpents, and they could only be cured by seeing the serpent. So too Christ is a sign of contradiction. He took away death by dying. He took away sin by suffering the greatest sin ever committed.
If anyone was bitten by a serpent, they looked at the bronze serpent on a pole and they were saved, but the bronze serpent was only a symbol. Even if the people were saved from the bite, they died eventually. Jesus Christ is the reality which was symbolized. When we look upon Jesus, and we believe in him, we are saved forever.
This difference is important. The symbol worked partially, and it worked simply by looking. The reality works fully, but merely looking is not enough. We must believe also. What do we have to believe? Jesus says, “If you do not believe that I AM, you will die in your sins.” The Pharisees want him to finish the sentence. They ask, “Who are you?” Jesus repeats the phrase again, “I AM.”
This is how God refers to himself. When Moses asked God for his name, God said, “I am who am.” God was saying that he does not participate in the universe. He is the foundation of the universe. He is the one essential fact. Everything else could have not existed, but God just IS. He was never created. There never was when he was not. There were a lot of things when I was not. The world got along just fine (at least, as fine as it ever has) before I came along. Back then, I did not exist, but many people did.
Jesus is telling the Pharisees that he has always existed and will always exist. When this Earth did not exist, he did. When this Earth stops existing, he will continue existing. This is what we must believe when we look at the Cross. We see, with our eyes, a man, totally defeated, a dead man, but we believe in our hearts that that man could never be defeated. We believe that he will win every battle. If we see him dead, we can know for certain that he has just gone to fight death, and he will emerge victorious.