The Apostle John was the last of Jesus twelve apostles to die and he lived to be an old man. He wrote this gospel toward the end of his life. By then, the other three gospels had been written, accepted and widely circulated. John’s purpose was two-fold. He was intentionally recording teachings and incidents that were not included in the other gospels, or writing them from a different perspective. And he was writing to counter a heresy that had sprung up in the early church called Gnosticism.
His main audience was primarily non-Jews who had become Christian. In his own words John says, “These things are written that you may believe that Jesus is the Messiah, the Son of God, and that by believing you may have life in His name.” (John 20:30)