And the LORD said unto Moses, Make thee a fiery serpent, and set it upon a pole: and it shall come to pass, that every one that is bitten, when he looketh upon it, shall live.

Wesley's Notes

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.

Matthew Henry's Concise Commentary

21:4-9 The children of Israel were wearied by a long march round the land of Edom. They speak discontentedly of what God had done for them, and distrustfully of what he would do. What will they be pleased with, whom manna will not please? Let not the contempt which some cast on the word of God, make us value it less. It is the bread of life, substantial bread, and will nourish those who by faith feed upon it, to eternal life, whoever may call it light bread. We see the righteous judgment God brought upon them for murmuring. He sent fiery serpents among them, which bit or stung many to death. It is to be feared that they would not have owned the sin, if they had not felt the smart; but they relent under the rod. And God made a wonderful provision for their relief. The Jews themselves say it was not the sight of the brazen serpent that cured; but in looking up to it, they looked up to God as the Lord that healed them. There was much gospel in this. Our Saviour declared, Joh 3:14,15, that as Moses lifted up the serpent in the wilderness, so the Son of man must be lifted up, that whatsoever believeth in him, should not perish. Compare their disease and ours. Sin bites like a serpent, and stings like an adder. Compare the application of their remedy and ours. They looked and lived, and we, if we believe, shall not perish. It is by faith that we look unto Jesus, Heb 12:2. Whosoever looked, however desperate his case, or feeble his sight, or distant his place, was certainly and perfectly cured. The Lord can relieve us from dangers and distresses, by means which human reason never would have devised. Oh that the venom of the old serpent, inflaming men's passions, and causing them to commit sins which end in their eternal destruction, were as sensibly felt, and the danger as plainly seen, as the Israelites felt pain from the bite of the fiery serpents, and feared the death which followed! Then none would shut their eyes to Christ, or turn from his gospel. Then a crucified Saviour would be so valued, that all things else would be accounted loss for him; then, without delay, and with earnestness and simplicity, all would apply to him in the appointed way, crying, Lord, save us; we perish! Nor would any abuse the freeness of Christ's salvation, while they reckoned the price which it cost him.