The book of John comes from the perspective of a direct disciple of Jesus Christ.
Of the four gospels, John and Matthew come to us directly from witnesses, Luke as a compilation of testimonies of witnesses, and Mark which came via Peter (a direct witness).
Each of the four gospels was written with a different intended audience, though we can relate to all. Matthew was intended for the Jews, Mark for the Gentiles, Luke writes to Theophilus and John seems to address the world at large. He tells us in John 10:31 that his point is for all of us to believe, "that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name." John was the last living of the twelve apostles. John had been imprisoned for his belief and was in exile at the time this gospel was written. He had seen a great deal of Christian persecution.
John begins with a particularly beautiful analogy which helps us to understand the wonder and origin of Jesus. These words also tell us that there were those human beings, Pontius Pilot, the Pharisee and others who persecuted believers, who did not understand Jesus as the light of the world but as a threat due to their limited understanding or choice to remain in darkness. John 1:1-5 states, "In the beginning was the Word, and the Word was with God, and the Word was God. He was with God in the beginning. Through him all things were made: without him nothing was made that has been made. In him was life and that life was the light of men. The light shines in the darkness, but the darkness has not understood."
John refers to the term "light" often when he recalls Jesus' words. For example, in John 8:12 Jesus states, "I am the light of the world. Whoever follows me will never walk in darkness, but will have the light of life." And, later in John 12:35, Jesus tells the disciples "You are going to have the light just a little while longer. Walk while you have the light, before darkness overtakes you. The man who walks in the dark does not know where he is going. Put your trust in the light while you have it, so that you may become sons of light."
Jesus further compares himself with water, which we all need to live, but Jesus is able to fill us up with the Holy Spirit which quenches our spiritual need. Jesus speaks in these ways so that people of His time could understand. In John 4:13 Jesus answers the woman at the well, "Everyone who drinks this water will be thirsty again, but whoever drinks the water I give him will never thirst. Indeed, the water I give him will become in him a spring of water welling up to eternal life."
Jesus words to Nicodemus come from John 3:16, "For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life." Jesus love for us, Gods' love for us is the common thread throughout all four gospels. Through different authors and witnesses the message is clear; 1. God really, really loves us in spite of what we've done. 2. He sent Jesus here to die for us so that we could start all over. 3. The love and forgiveness God offers is free through His grace. 4. We can accept, of our own free will, the love and forgiveness God offers by asking Jesus into our hearts. 5. We receive the gift of the Holy Spirit as a guide and comforter and a neat relationship with God/Jesus. 6. We also receive the gift of eternal life with God.
All the gospels, and all of the Bible for that matter, have important unique information about human nature, God love for us and the struggle between God and human nature. Jesus is the bridge to help us get past our own problems, serve others and serve God. God sent Jesus to let us know how we're supposed to live and love each other. He came as Jesus to let us know that He loves us dearly and wants us to freely choose to live with Him in eternity.