The gospel of John represents one piece in a four-part testimony of the life of Jesus. This is important because the Bible requires that there be two witnesses to establish a fact. The use of four gospels each based on testimony of eye witnesses makes the totality of the gospels very credible. The fact that each of the gospels is not identical to the others adds to the overall credibility.
If one were to interview four eye-witnesses to any event in history, each of them would emphasize different facts and provide a different presentation of the information depending on their audience. That is exactly what we find in the gospels – four unique but internally consistent testimonies of the life and ministry of Jesus Christ, a real man who lived a real life in Israel.
Looking at all four gospels, we see that each is addressed to a different audience, for example, the Gospel of Mark is written with a gentile (primarily Roman) audience in mind. Little detail of Jewish history or culture is included.
The Gospel of Matthew is written to the Jewish reader. Matthew starts off with a very detailed genealogy of Jesus and emphasizes Jesus as the long-awaited Messiah.
The Gospel of Luke was written to a gentile audience. In as much as Luke was a physician and educated in Greek, he writes a more detailed, carefully researched gospel focused on Jesus as the Son of Man a Savior sent to save the lost sinner.
The Gospel of John was written with a strong emphasis on the deity of Jesus Christ. John wrote to encouraged believers and to call unbelievers to faith in Jesus Christ