From A. W. Pink’s Exposition of the Gospel of John:
From what has been said, it will be evident that when God told Moses to make a serpent of brass, fix it upon a pole, and bid the bitten Israelites look on it and they should live, that He was preaching to them the Gospel of His grace. We would now point out seven things which these Israelites were not bidden to do.
6. They were told not to look at Moses. They had been looking to Moses, and urging him to cry to God on their behalf; and when God responded, He took their eyes from off Moses, and commanded them to look at the brazen serpent. Moses was the Law-giver, and how many today are looking to him for salvation. They are trusting in their own imperfect obedience to God’s commandments to take them to heaven. In other words, they are depending on their own works. But Scripture says emphatically, “Not by works of righteousness which we have done, but according to his mercy he saved us” (Titus 3:5). The Law was given by Moses, but grace and truth came by Jesus Christ, and Christ alone can save.
I don’t think I can add anything to this gem; how about you?