John Wesley on Numbers 21:8

A fiery serpent - That is, the figure of a serpent in brass, which is of a fiery colour. This would require some time: God would not speedily take off the judgment, because he saw they were not throughly humbled. Upon a pole - That the people might see it from all parts of the camp, and therefore the pole must be high, and the serpent large. When he looketh - This method of cure was prescribed, that it might appear to be God's own work, and not the effect of nature or art: and that it might be an eminent type of our salvation by Christ. The serpent signified Christ, who was in the likeness of sinful flesh, though without sin, as this brazen serpent had the outward shape, but not the inward poison, of the other serpents: the pole resembled the cross upon which Christ was lifted up for our salvation: and looking up to it designed our believing in Christ.