The Gospel of Matthew was written primarily for the Jewish reader. The record in Mark had the Roman reader in mind. Luke was prepared for a wider audience—the Greek-speaking world. But the Gospel of John has a wider focus than all, as it was written for the whole world.
John explains his purpose for writing this book in John 20:30–31, “And truly Jesus did many other signs in the presence of His disciples, which are not written in this book; but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in His name.” By his own admission, the purpose of this book is to tell the reader how to enter into eternal life: there is only one way—to believe in the Lord Jesus Christ. So critical is this theme in the book that the word “believe” occurs about 99 times in this Gospel alone. I will be focusing on this theme of believing for eternal life as we go through the book.
Where the other Gospel writers focused on Jesus’ royal lineage, or His humanity, John presents Jesus in His deity. The Gospel of John presents the signs and the sayings of Jesus. Another way to put it is that John writes about the works and the words of Jesus (John 5:20, 24). He is presented as the second member of the trinity, who became man in order to die for us on the cross (Phil. 2:5–8). This is the central emphasis of the book. This of course ties into John’s point, that to believe in Him is to receive the gift of eternal life.