Numbers 21:8

Many things were done in figure which were done in former times. For when he said that the fathers who were bitten by serpents in the desert could not be healed in any other way except that Moses hung up a brazen serpent, and when this was seen, those deadly bites and injurious effects of the poison were cured, he added: But these things were done in figure to instruct us (1 Corinthians 10:6). In the image, a bronze serpent was placed on a cross; because the true one to be crucified was announced to the human race, who would empty the venom of the devilish serpent, cursed in its image, but in truth, would erase all curses of the world.