Historicity
The Jesus Seminar has argued that verses John 14:30-31 represent a conclusion, and that the next three chapters have been inserted into the text later. This argument considers the farewell discourse not to be authentic, and postulates that it was constructed after the death of Jesus. Similarly, Stephen Harris has questioned the authenticity of the discourse because it appears only in the Gospel of John, and not in the Synoptic gospels. However, scholars such as Herman Ridderbos see John 14:30-31 as a "provisional ending" just to that part of the discourse and not an ending to the entire discourse.
Fernando Segovia has argued that the discourse originally consisted on just chapter 14, and the other chapters were added later, but Gary M. Burge opposes that argument given the overall theological and literary unity of the work and that the discourse has much in common with the gospel as a whole, e.g. the themes of Jesus death and resurrection and his care for his own.
In 2004 Scott Kellum published a detailed analysis of the literary unity of the entire farewell discourse and stated that it shows that it was written by a single author, and that its structure and placement within the Gospel of John is consistent with the rest of that gospel.
Read more about this topic: Farewell Discourse