The Israelites journeyed in the wilderness forty years after leaving Egypt. They endured various hardships, one being venomous snakes that killed many of them. God told Moses to make a bronze snake on a pole.
Moses did, and “if a serpent had bitten anyone, when he looked at the bronze serpent, he lived” (Num. 21:9). The bronze snake was preserved, and later turned into an idol, for 2 Kings 18:4 reports that the Israelites had been burning incense to it.
Jesus referred to the bronze snake on one occasion: “As Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up” (John 3:14).
The early Christians probably noticed the parallel: Moses’ bronze snake, lifted up on a pole, saved people from death, and Jesus, lifted up on a cross, saved people from eternal death.