The Basis for the New Birth (John 3:14-21)

Christ had to die (vv. 14–17).

Christ again refers Nicodemus to the OT, this time Num. 21, the account of the brazen serpent. The serpents were biting the Jews and killing them, and the strange solution to the problem was found when Moses made a serpent of brass! Looking to the serpentin faith brought healing. In like manner, Christ was made sin for us, for it was sin that was killing us. As we look to Christ by faith, we are saved.
Did all this happen for judgment and condemnation? No. That was never God’s purpose. 
Notice how central Jesus is to the passage. Verse 15 emphasizes the words “in him” and they appear again in verses 16–18, while verse 17 talks about God’s saving the world through him. 
Christ had to die before men could be born again;
His death brings life. 
What a paradox!

Sinners have to believe (vv. 18–21).

Faith in Christ is the
only means of salvation. God’s command to Moses in Num. 21 was not that he kill the snakes, make a salve for the wounds, or try to protect the Jews from being bitten. It was that he lift up the brazen serpent and tell men to look by faith
Not to look meant condemnation; faith meant salvation.
John here goes back to 1:4–13, the symbolism of light and life, darkness and death. Sinners not only live in darkness, but they love the darkness, and refuse to come to the light where their sins will be exposed and can be forgiven.
And let us not miss the word “
” in John 3:18. We exist in a state of condemnation by birth—our sin natures inherited from Adam.
We enter physical life in a state of spiritual death and then our sin natures compound the problem with sinful behavior. 
In John 3:19–21 we see the verdict.
People reject Christ because of evil deeds and because they hate the light
. God does not label their deeds evil because they love darkness; they love darkness because that is their very nature. 
Five times John mentioned light, a word he had already introduced in chapter 1. James Boice illustrates this by using the familiar image of a father and small child throwing a ball. No matter how close they stand to each other, the child’s awkward throw goes awry. The problem is not the distance between them but the child’s aim. Well-meaning people, even those who seek righteousness in their own understanding, fail—that is why we need the cross.
We dare never forget how Isaiah put it:
For all of us have become like one who is unclean, And all our righteous deeds are like a filthy garment; And all of us wither like a leaf, And our iniquities, like the wind, take us away. There is no one who calls on Your name, Who arouses himself to take hold of You; For You have hidden Your face from us And have delivered us into the power of our iniquities.” (Isaiah 64:6–7).
My friends, every human being has a choice—eternal life or eternal death. Choose life! Believe in Jesus unto salvation.
“For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life. “For God did not send the Son into the world to judge the world, but that the world might be saved through Him. (John 3:16–17)