It is well known that the gospel of John is considerably different than the other three Gospels. One of the reasons that the Gospel of John seems so different is that the three synoptic gospels are so similar. Because of the similarities between Matthew, Mark, and Luke some theory of literary dependence must be given to explain the close relationship.
There are several extended dialogues which have no real parallel in the synoptic gospels. Jesus does not re-interpret the Mosaic law, as in the Sermon on the Mount, nor does he predict the fall of Jerusalem (cf. Mark 13 and parallels.) In fact, there is no prediction of a second coming in John, although Jesus does promise to send the Paraclete to the disciples after he returns to heaven (14:25-26, 16:7-15). The Last Supper is not described as an ongoing celebration, rather, John describes Jesus washing the feet of the disciples (13:1-16). While the arrest and crucifixion is described in similar ways to the synoptic gospels, there is no agony in the garden of Gethsemane.
I am following Andreas J. Köstenberger, A Theology of John’s Gospel and his Letters (Grand Rapids, Mich.: Zondervan, 2009). Köstenberger follows B. F. Wescott’s observation that John’s Gospel was written after the success of the (Pauline) Gentile Mission, after the destruction of Jerusalem, and at the same time as the emergence of Gnosticism as competitor to Apostolic Christianity.
For Köstenberger, the Fall of Jerusalem is the most important factor. I am sure that the rise of Gnosticism is a major factor, but I am not sure that the success of the Gentile mission is as much of a factor than sometimes assumed. John wrote the gospel some thirty years after the death of Paul, from Ephesus, the city where Paul had his most success among Gentiles. Yet the Gospel has very little to say about Gentiles. The Samaritan Woman (John 4) is a possible example, but Samaritans are a in many ways neither Jew nor Gentile. The healing of the official’s son in John 4:46-54 is sometimes offered as an example of a Gentile who encounters Jesus, but if he is John certainly does not make this explicit.
On the one hand, the Gospel is evangelistic. John wrote to Jewish readers who might be open to Jesus as an alternative to the Temple and the festivals. But there are a few stories which are could be described as drawing Gentiles to Jesus. The story of the blind man who is healed in John 5 may show that Jesus is superior to Asclepius, a Roman god of healing. Given the number of allusions to the Hebrew Bible and the importance of the Jewish story of redemption, it is clear that the main target of the Gospel is Jewish.
On the other hand, the Gospel is apologetic. John wrote to Christians (either Jewish or Gentile) in order to clarify who Jesus was as an answer to growing questions raised by developing Gnostic theology. There is a serious theological challenge developing in the church, John must address this as insufficient for explaining who Jesus was. John describes Jesus as the Word, equal with God because he is God. But Jesus is also flesh, fully human. These two facts are stated in the prologue and supported throughout the Gospel of John.
The Gospel of John is therefore a window into the end of the apostolic era. Christianity was making progress against paganism, but needed to to develop a theology of Jesus in the face of an internal challenge. Can we draw other implications from the differences between John and the Synoptics?