The gospel of John focuses on one key question: Is it possible to belive that Jesus was the Son of God in flesh and blood.
It he wasn't, we can quit right here and now. But if he was God's Son, we must shape our entire life around this amazing story.
John said it best, "These things are written so that you may belive that Jesus is the Christ, the Son of God and that by believing, you may have life in his name." (John 20:31)