16 ...whosoever believeth in him should not perish, but have everlasting life.
17 ...that the world through him might be saved. 18 He that believeth on him is not condemned:
Jesus explained to Nicodemus, the one He was talking with in these verses, that he needed more than his birth and his good deeds to enter the Kingdom of God. If birth to a good family, and good deeds could open the doors of heaven to Nicodemus, then there would have been no need for Jesus to come... and die.
Jesus gave Nicodemus a clear explanation of the motivation behind His coming. The motive was love. God’s love moved Jesus to act for the benefit of a fallen world. His actions established an infallible way for Nicodemus and the rest of us, who have fallen short of God's requirements for our lives, to gain permanent, everlasting reconciliation with God.
This permanence was promised by Jesus to Nicodemus, and any other person who would trust in Jesus and His great gospel work. Jesus deliberately used forms of words that leave no doubt about the solid nature of His promises. Read these verses above again. He used the present possessive “have” when speaking of His promise of eternal life.
At the moment of faith a believer possesses “eternal life.” Since it is eternal it has no end, so once gained it cannot be lost. The legal penalty for sin has been permanently lifted. A believer can never be tried in the courts of heaven, because Jesus has paid his/her legal debt to God by His death on the cross. When a person believes Jesus died for his/her sins, and He defeated death by His resurrection from the dead, that person is freed from the status that was theirs because of sin.
Jesus talked to Nicodemus face to face. There was no one between them. No one to add something to what Jesus said, or tell Nicodemus he must join their church to gain the benefits of Jesus' gospel. Just Jesus and Nicodemus... This is pure Christianity... just Jesus and a person He came to save. Don't let anyone get between you and Jesus. Simply trust in His promises and take Him at His Word.