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The concept of the serpent is very misunderstood. People cannot explain why Moses put a brass serpent on a pole or why Jesus compared himself to that serpent. Perhaps more mysteriously (if I may go New Agey on people), Messiah and serpent both equal 358 using standard gematria.
Moses was instructed by God to put the brass serpent on a pole, in my view, because it was the serpents which bit them and poisoned them, and was now being used by God to heal them. God fixed them, like no other could.
Jesus compared Himself to that serpent, because He was linking their belief at the time of Moses, to Him now - healing them, from death, in this context. God gave a manner by which to be healed, with Jesus once-and-for-all, like back with Moses and the brass serpent on a pole. Serpents were big in Egypt, remember the serpent in Ezekiel 7:12 "For they cast down every man his rod, and they became serpents: but Aaron's rod swallowed up their rods." To me, this is saying, that God outdoes what others think is greater (like the brass serpent being greater than serpents that bit Moses' people (healed their poison), like Jesus is greater than the brass serpent, etc).