Jesus made reference to Moses' lifting up of the copper serpent in the Wilderness.

John 3:14-15 14 And just as Moses lifted up the serpent in the wilderness, so the Son of man must be lifted up, 15 that everyone believing in him may have everlasting life.

That reference was from:

Numbers 21:7-9 7 Finally the people came to Moses and said: "We have sinned, because we have spoken against Jehovah and against you. Intercede with Jehovah that he may remove the serpents from upon us." And Moses went interceding in behalf of the people. 8 Then Jehovah said to Moses: "Make for yourself a fiery snake and place it upon a signal pole. And it must occur that when anyons to look at it and so must keep alive." 9 Moses at once made a serpent of copper and placed it upon the signal pole; and it did occur that if a serpent had bitten a man and he gazed at the copper serpent, he then kept alive.

When they looked upon the copper serpent they were saved. That is a prophecy leading to our looking to Jesus likewise who had died upon a pole. Our putting faith in him will save us.

However the copper serpent did not end there.

2 Kings 18:1-5 18 Hez·e·ki'ah the son of A'haz the king of Judah became king. 3 And he continued to do what was right in Jehovah's eyes, according to all that David his forefather had done. 4 He it was that removed the high places and broke the sacred pillars to pieces and cut down the sacred pole and crushed to pieces the copper serpent that Moses had made; for down to those days the sons of Israel had continually been making sacrificial smoke to it, and it used to be called the copper serpent-idol. 5 In Jehovah the God of Israel he trusted; and after him there proved to be no one like him among all the kings of Judah, even those who had happened to be prior to him.

For centuries the copper serpent, something that saved them once, was being used as an idol. The same is going for the instrument that people believe Christ died upon. They worship the cross by wearing it as a necklace, or earrings. People have it in their churches, houses, etc. But it is still idolatry just as it was back then.