The brass serpent on the pole was not just the cross. When Jesus was crucified, it was confirming what Deut. 21:22-23 states…”If a man has committed a sin worthy of death and he is put to death, and you hang him on a tree, his corpse shall not hang all night on the tree, but you shall surely bury him on the same day (for he who is hanged is accursed of God), so that you do not defile your land which the Lord your God gives you as an inheritance.” Now, Christ never committed the first sin, but when He was imputed His sheep’s sins, God treated Him as if He had personally committed them. This is exactly what imputation teaches. He was not literally guilty of sinning, but was legally declared guilty of sinning. And the brass snake typified that.