Prologue to the Gospel according to John
What Jesus taught and what he did are tied inseparably to who he is. John shows Jesus as fully human and fully God, the Son of God. Although Jesus took upon himself full humanity and lived as a man, he never ceased to be the eternal God who has always existed, the Creator and Sustainer of all things, and the source of eternal life.
This is the truth about Jesus, and the foundation of all truth. If we cannot or do not believe this basic truth, we will not have enough faith to trust our eternal destiny to him. That is why John wrote this Gospel: to build faith and confidence in Jesus Christ so that we may believe that he truly was and is the Son of God (John 20:30, 31).
John wrote to believers everywhere, both Jews and non-Jews (Gentiles). As one of Jesus' twelve apostles, John was an eyewitness, so his story is accurate. His book is not a biography (like the book of Luke); it is a thematic presentation of Jesus' life, with greater emphasis on the principles of His teaching. Many in John's original audience had a Greek background. Greek culture encouraged worship of many mythological gods, whose supernatural characteristics were as important to Greeks as genealogies were to Jews. John shows that Jesus is not only different from, but superior to these gods of mythology.
And many other signs truly did Jesus in the presence of his disciples, which are not written in this book: (John 20:30)
But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name. (John 20:31)