Note that Nicodemus (verse 2) does not ask whether Jesus was God (He was - see John 8, John 10 30 ).
Jesus knows heart of very man (1 Sam 16 7 , also John 2 23-24 ).
If one cannot believe what the earthly things say (v 12a) then how can one believe the heavenly (v 12b).
Compare this with Romans 1 20 , Psalm 19.
Verse 12 may indicate: if you don't believe the earthly - new birth (because it pertains to us) (v 3), then you will not believe the heavenly (because it pertains to Christ) - salvation is by faith (v 13-21)
Salvation is heavenly (v 13). Jesus is more than a man that "God is with" (see v 2), He is God (This is what John's Gospel proves). Moreover, no one has ever entered heaven (up to this point - See Acts 2 34 ). What Jesus is saying is: "Know who it is to whom you are speaking. I am not merely a teacher come (or sent) from God. I am indeed the Messiah, the Son of man, foretold in Daniel".
Due to Israel 's wickedness, snakes (serpents) were sent into the wilderness camp and many died. A bronze serpent was moulded and hung on a stick which was raised. Those looking upon it lived (See Numbers 21 1-9 ).
(Moses is who the Pharisees trusted (See also John 5 45-46 , John 9 28 )).
Christ came to die (John 6 38 etc) - by being lifted up on the cross so "all may behold him".
Whoever , that is, any one, without exception (see Romans 4).
Believes in Him is the one act of the soul to have an interest in Jesus. It is the belief of the heart and will. Believing is "heart-looking". Without looking at the bronze serpent the Israelis died.
Should not perish but have eternal life (See also Rev 20 11-14 ). A saved man can but die once; an unsaved man dies twice - (Spurgeon).
God so loved the world. (Rom 5 8 )
He gave His only begotton Son. Sent (gave) by God to be the Saviour (John 5 23 , 1 John 4 14 ) (Phil 2 5-11 ), delivered for our offences (Rom 4 25 , 8 32 ).
Note Jesus clearly has answered Nicodemus by this point. He (Jesus) has referred to himself as "the only begotton Son of God" and the "Son of Man", hence Jesus was from God and was God.
That whosoever believeth shall not perish but have eternal (everlasting) life. See verse 15
Note that the question is never "Did Christ die for me?" but always "do I believe in Christ?"
(v 17) Jesus came to receive the punishment for our sins, not condemn to world. He justifies believers by His blood, saving us from His wrath (Rom 5 9 ).
(v 18) A man who believes, will have eternal life, because he is not condemned (an unsaved man is condemned because the wages of sin is death ) (1 Tim 4 10 ). If you believe your condemnation has been dealt with, then you believe the punishment has been borne by Jesus Christ on the cross (Rom 8 3 ). Conversely if you don't believe you live on in your condemnation.
"Not condemned" says "he is pardoned, acquitted, justified, cleared from all guilt, believed from the curse, no longer counted a sinner, reckoned righteous" .
(v 19-21) Men have always, and will always love darkness due to corruption of the human nature - their deeds were evil (v 19) . A light shows up darkness. Men and women are clearly separated into two classes along moral grounds - those in darkness and those in light.
He that lives by truth (NIV) ie truly converted is in the light. Hence, if you are a seeker of the truth, the only way to find it is to believe on Him who died for you. Without the light you are condemned and will suffer punishment by eternal fire in hell.
 It is the blood of Christ that justifies us. The value in the blood is that it makes me clear of all guilt in God's sight (something the sacrifices of Aaron could never do - see Hebrews - we have no more consciousness of sins (Heb 10:2)). That is, God imputes no sin, because the price has been paid in full. Viewed another way, God imputes a sense of righteousness, and in such a person justification is fully recognised. Given that our natural relationships have failed, God meets these with the precious blood of Jesus that justifies us. God therefore sees us as having not failed in this relationship - hence one can say justification is just-as-if-it-hadn't-happened. Since we are now held as not having had failed in the relationship with God we have righteousness - of course not of our selves - it rests entirely with Christ. That is, our relationship is not based on what I do, but on the work of Christ, whereby Christ was glorified by God.
 Ryle, J.R. (1982) Ryle's Expository Thoughts On The Gospel of John, vol 3, Baker House, pg194