"Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness. If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things? And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven." John 3:11-13
This first verse has the meaning of speaking of that which we know, and testifying to that which we have seen. This is clearly brought out in the presence of relative pronouns in the Greek. Jesus is bringing to bare the fact that the leadership of Israel had, for the most part, rejected all that was shown them by Christ and His witness, John. It is a dangerous thing to knowingly and willfully disregard truth that is very obvious to everyone. At this point, Christ's miracles, deity, and preaching were not accepted by most of the nation.
He then asks a very simple question, "If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things?" Those who reject the most basic truths of God's Word can never hope to understand and expound upon the more spiritual things, and yet, this is what most "theologians" do. They reject the faith-based approach to Scripture. They question the Bible's foundation, history, preservation, and inspiration, and then expect to be able to explain its deep spiritual truths. Even more sad is the fact that many read after them, hoping to find the answers to difficult passages, only to be tainted by their ignorance and lack of spiritual perception. Beware, that you do not reject the smallest truth that God's Word has to offer. Truth builds upon truth.
Christ is encouraging Nicodemus to change his unbelieving heart because He is about to reveal to Him that no man is qualified to be the Savior, but the Son of God. He says, "And no man hath ascended up to heaven...." The Lord is quoting a portion of Proverbs 30:4 which says, "Who hath ascended up into heaven, or descended? who hath gathered the wind in his fists? who hath bound the waters in a garment? who hath established all the ends of the earth? what is his name, and what is his son's name, if thou canst tell?" This Old Testament verse is prophetic of the Son of God, and Christ uses it to point to His deity and preeminence. No man has ascended to heaven. None has the ability to redeem himself or his brother. Yet there is hope. Jesus goes on, "but he that came down from heaven...." One, qualified to redeem mankind has arrived! Nicodemus is looking right at Him. Then, in one concise statement, Jesus boldly declares both His humanity and His deity by saying, "...even the Son of man which is in heaven." The Son of man speaks of His humanity. Only a man could suffer for the sins of a man (Isaiah 53). Only a man could experience the trials and temptations that man faces (Hebrews 4:15). Only a man could shed his blood for men. But yet, this man must be faultless, unable to be tempted with sin, mighty to save, able to conquer sin and death, sovereign over all! This man must be God. The last part of this verse declares His deity, "...even the Son of man which is in heaven." This last part of the verse is completely excluded from the Critical Text. Literally translated, it would say, "The Son of man, the one who is being in heaven." Inexplicable as it is, Christ was in heaven as He sat there in the flesh talking to Nicodemus. No surprise really. How can God be pulled apart? Though Jesus took on human flesh, He never for one moment laid aside any attribute of His deity. He is God! He is qualified to be the Savior of mankind, because none other is both man and God. No other is sinless and yet human, save our blessed Lord Jesus Christ.