The serpent, which bit and poisoned the Jews is sin, death and an evil conscience. I know that I must die and that I am under the power of death; I cannot free myself and must remain in this state until a dead serpent is set up for me, one which, because it is not alive, can harm no one, but rather benefit, as did the serpent of Moses. Now, this is Christ. I see him hanging on the cross, not beautiful nor greatly honored. If his death upon the cross were in fashion to win for him the plaudits of men, the flesh might say that he deserved his honors and his exaltation by his works. But I see him hanging in disgrace on the cross, like a murderer and malefactor; thus, reason must say that he is cursed before God. The Jews believed that this was true and they could only consider him the most cursed of all men before God and the world, for they remembered this passage in the Law of Moses: “He that is hanged is accursed of God.” (Deuteronomy 21:23)
Moses had to set up a serpent of brass, which looked like the fiery serpents, but did not bite or harm any one, nay, it rather saved the people. Thus, Christ also has the form and the appearance of a sinner, but has become my salvation; his death is my life; he atones for my sin and takes away from me the wrath of the Father. The living, fiery serpent is within me, for I am a sinner, but in him is a dead serpent; he was indeed regarded a sinner, but he never committed any sin.
If, then, man believes that the death of Christ has taken away his sin, he becomes a new man. The carnal, natural man cannot believe that God will gratuitously take away and forgive us all our sins. Reason argues in this manner: You have sinned, you must also atone for your sin. Then it invents one good work after another and endeavors to take away sin by good works. But the Gospel of Christ is: If you have fallen in sin, another must atone for you, if a man believes this, he becomes one with Christ, and has everything that is Christ’s.
(John chapter three), then, signifies that our works are nothing, and that all human power can do is useless, but faith in Christ does it all.
Martin Luther, First Church Postil for the Feast of the Holy Trinity (John 3:1-15)