When Nicodemus came to Jesus, Jesus told him plainly, "Most assuredly I say to you, unless one is born again, he cannot see the kingdom of God." Again Jesus told him, "Most assuredly I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God."
Jesus makes it clear that getting into heaven isn't something that just automatically applies to everyone. Instead, He says, you have to be born again, born of the Holy Spirit.
How does that happen? Jesus explains with an illustration from Jewish history, with which Nicodemus would have been very familiar. He said, "As Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up, that whoever believes in Him should not perish, but have eternal life."
The story Jesus is referring to is found in Numbers 21. The Israelites had sinned against God by grumbling and complaining against Him, and by refusing to trust in Him. So God sent poisonous snakes among the people, and many were bitten and died. Then they realized that they had sinned, and asked Moses to pray for the snakes to be taken away. Instead, God told Moses to make a bronze snake and put it up on a pole, so that anyone who was bitten could look at it and live.
As one of the religious leaders in Israel, Nicodemus would have clearly understood that it was the sin of the people that led to their death, and it was this from which they were being rescued. Jesus is drawing the same parallel: that our sin — and there is no one who does not sin — leads to death — that is, eternal separation from God (read "hell") — but by faith in Jesus, we can be rescued and have eternal life (read "heaven").
Jesus goes on to explain, "For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish, but have everlasting life."
This is God's plan, and His desire for you, that through faith in Jesus, you should be delivered from sin and receive eternal life.
Jesus says that this gift of eternal life is available to "whoever believes in Him". That's the key — to believe in Jesus, to trust in Him, to rely on Him completely. On another occasion, when the Jews had asked, "What must we do to do the works of God?" Jesus replied, "This is the work of God, that you believe in Him whom He sent."
What, then, does Jesus say about unbelief? Returning to His conversation with Nicodemus, Jesus went on to say, "He who believes in Him is not condemned, but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God. And this is the condemnation, that the light has come into the world, and men loved darkness rather than light, because their deeds were evil. For everyone practicing evil hates the light and does not come to the light, lest his deeds should be exposed."
Jesus didn't just say that "light" had come into the world; He said "the light", referring to Himself. He would later declare, "I am the light of the world." To love darkness rather than the light simply means to prefer your sin over God. And most people in this situation not only don't want their sin to be exposed, they don't even want to admit that it's sin. Jesus makes it clear that unbelief is not an intellectual, but a moral problem.
This is what Paul was talking about in his letter to the Romans. He writes, "For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, because what may be known about God is plain to them, for God has shown it to them. For since the creation of the world, God's invisible qualities — His eternal power and divine nature — have been clearly seen, being understood from what has been made, so that people are without excuse."
"Unrighteousness" can be understood simply to mean "not right". It's not right to suppress the truth. There is plenty of evidence available, both for the existence of God, which we have in His creation, and for the divinity of Jesus, which we have through the testimony of those who were witnesses to Jesus' life, death, and resurrection. To deny, ignore, or avoid this evidence is to suppress the truth. Jesus explains that the reason people do this is because they prefer their sin over God. Rather than admit sin, come to Jesus, and have eternal life, it's much more comfortable (for some) to craft one's own spiritual worldview that condones one's own sin and eliminates the need for repentance.
But Jesus clearly says that this self-deception leads only to death. He told the people, "If you do not believe that I am the one I claim to be, you will indeed die in your sins." The apostle John records Jesus' words, "I am the Alpha and the Omega, the Beginning and the End. I will give of the fountain of the water of life freely to him who thirsts. He who overcomes shall inherit all things, and I will be his God and he shall be My son. But the cowardly, the unbelieving, the vile, the murderers, the sexually immoral, those who practice magic arts, the idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death."
The Psalmist writes, "The Lord takes delight in His people; He crowns the humble with salvation." It takes humility to renounce one's unbelief and to confess one's sin, but the rewards — being delivered from sin and death and forever abiding in God's love — are priceless.
Scripture references: John 3:3, John 3:5, John 3:14-15, Numbers 21:4-9, John 3:16, John 6:28-29, John 3:19-20, John 8:12, Romans 1:18-20, John 8:24, Revelation 21:6-8, Psalm 149:4