Jesus began His conversation with Nicodemus by telling him that he must be born again in order to be a part of the Kingdom of God and to have a home in heaven. The second part of Jesus’ conversation with Nicodemus has to do with how one can be born again. The answer to this question is both simple and complex: you must believe.
Before we examine our text, we must remind ourselves that being born again is a work of God. There is nothing that you and I can do to save ourselves. Regeneration is work wrought only by the Holy Spirit. The moment you receive Jesus as your Lord and Savior, you are immediately baptized in the Holy Ghost. You are indwelled by the Holy Ghost. It is in that very moment that you are born again. However, in order for the Spirit of God to do the work of regeneration in you, there is one requirement and that is that you must believe. In our text, we find that there are several things that we must believe concerning Jesus in order to be born again.
The authority of Jesus (v. 13). Verse 13 is tied closely to verse 12. Jesus told Nicodemus that the truth that you must be born again is a spiritual truth. It a truth that came from heaven. In verse 13, Jesus tells Nicodemus that he could trust what He was saying because He has the authority to speak concerning heavenly things. Jesus has this authority because He is the only one who has come from heaven down to earth. “Jesus insists that no-one has ascended to heaven in such a way as to return to talk about heavenly things. Only in heaven can true wisdom be found (cf. Pr. 30:4). But Jesus can speak of heavenly things, not because he ascended to heaven from a home on earth and then descended to tell others of his experiences, but because heaven was his home in the first place, and therefore he has ‘inherently the fulness of heavenly knowledge’”.  It is important to note the authority of Jesus in order to believe in Him. If Jesus did not speak with such authority, then why would anyone believe?
The exaltation of Jesus (v. 14). Jesus gives an illustration in making a prophecy concerning Himself. Nicodemus was well versed in the Old Testament scriptures. Jesus uses an Old Testament event to reveal His true identity as the Messiah, the Savior of the world. Look at what the scripture says in Numbers 21:5-9. The people of Israel were bitten by terrible serpents or snakes. God tells Moses to put a serpent on a pole and lift it up. All those who look at the serpent on the pole were healed from the snake bite. Jesus uses this passage to illustrate the fact that all of us have been bitten by the curse of sin. As Romans 3:23 says, “All have sinned and fallen short of the glory of God.” Because we have been bitten by the curse of sin we are all destined to experience eternal death. Romans 6:23 says, “For the wages of sin is death….” Jesus then explains to Nicodemus that just as the people of Israel was healed when they looked upon the serpent; all those who look to Him will be saved from the curse of sin. Jesus was speaking here concerning the cross. It is on the cross that Jesus is exalted. In order to be saved from the curse of sin, we must look to the cross.
The power of Jesus (v. 15-21). Because Jesus has heavenly and authority and because He is lifted up on the cross, He has the power to save all those who come to Him by faith and believe. There are several things we see here that Jesus has the power to do. (1) He has the power give eternal life (v. 15-16). Sin brings eternal death, Jesus gives eternal life. This is where free will comes in. You and I are cursed because of sin, therefore, we are bound for eternal death (hell). However, because of what Jesus did for us on the cross, we can be set free from sin and death. If we choose to believe on the Lord Jesus, we are given the gift of eternal life. Jesus has already done everything that must be done in order to save us from the curse of sin. He has paid the price of sin for us. He was lifted up on the cross. All we must do is make the choice to believe in Him. (2) He has the power to free us from condemnation (v. 17-19). Those who have not believe on the Lord Jesus are condemned already. Think about it. You and I were born in sin. We have inherited a sin nature. In other words, you cannot choose to be condemned. You are already condemned. Condemnation is the natural result of living in a sin cursed world. So, to be condemned, there is nothing that you have to do. Just go on as you are. As a matter of fact, you do not have to commit a sin in order to be condemned. You were a sinner before you ever sinned. You are condemned already. However, there is good news. Jesus came, not to condemn, but to save. If you believe on the Lord Jesus, you will be saved from condemnation. As Romans 8:1 says, “There is no condemnation for those who are in Christ Jesus…” (3) He has the power to give light (v. 19-21). Jesus is the light of the world. That light reveals to us our sinfulness and our need for a Savior. Many people remain condemned in their sin because they love darkness more than light. They enjoy their sin too much. They refuse to repent and turn to the light. You see, in order to be born again, you must repent of your sin. You must turn away from the darkness and you must turn to the light of the glorious Gospel of Jesus Christ.
The greatest decision you will ever make is the decision to believe on Jesus. He alone has the power to set you free and give you eternal life. In order to have eternal life in heaven, you must be born again. In order to be born again, you must believe.
 Carson, D. A. (1991). The Gospel according to John (pp. 200–201). Leicester, England; Grand Rapids, MI: Inter-Varsity Press; W.B. Eerdmans.