The last discourse of Jesus to his disciples in chapters 14 through 17 in the Gospel of John is striking in its uniqueness. It is unlike the teaching of Jesus in any of the Gospels, even surpassing the distinctive speech of Jesus in the rest of John's Gospel. Besides the intimacy of this discourse, there is an atmosphere of heavenly mystery which encompasses all of it.
There is a speculative theory that the discourse in John 14-17 actually comes from the time when the risen Jesus appeared to his disciples. As Luke says, "After his suffering he presented himself alive to them by many convincing proofs, appearing to them during forty days and speaking about the kingdom of God" (Acts 1:3). Could it be that John 14-17 represents this period of the mysterious "forty days" when the risen Jesus talked with his disciples about "the kingdom of God?" The author of this Gospel is not above putting things out of place: John puts Jesus' cleansing of the Temple at the beginning, not the end, of his Gospel for theological purposes. He leaves an impression that John 14-17 is an interrruption of his narrative by surpising us with this discourse between the last supper and the arrest of Jesus. Moreover, most of Jesus' discourse seems to fit the post-resurrection period when Jesus comes to them, but is not yet ascended to his Father (20:17). Jesus speaks of having been with them as if his earthly life is behind him, and as if he is in an inbetween time of having been with them and going to be with his Father (16:4-5). It is easy to consider John 14-17 as a disclosure of the teaching of the risen Jesus to his disciples inserted in the narrative as if it were his teaching just before his death. Of course, the reason to insert this discourse here is to avoid the sense of an anti-climax by placing this long discourse following the drama of the crucifixion and resurrection of the Lord.
Others would object to this theory as perhaps being odd and claiming too much. They would explain the strange use of tense, e.g. speaking in the present as if it were past, as a method of preparing them for the time when Jesus would no longer be with them bodily. In The Gospel According to John, XIII-XXI (Doubleday, 1970, p. 581 ff.) Raymond E. Brown cited the opinion of a scholar named Hammer that John 15-16 represents what was said by the risen Jesus in his appearance before the five hundred (I Corinthians 15:6) ,while John 17 was spoken in Jesus' final appearance before his ascension. Brown's opinion is that "parts of the Last Discourse do have a post-resurrectional air" and that we cannot exclude the possibility that it contains sayings which "may have originally been transmitted in a post-resurrectional context," but he cannot accept Hammer's theory completely because the theory is not necessary to explain how Jesus' sayings from different contexts are woven into the narrative and how the life of Jesus is always seen in the light of Easter.
Regardless of the opinions of the scholars, we might conduct a thought-experiment and meditate on John 14-17 as a post-resurrection narrative. If we do, we shall better understand the message of Jesus to his apostles about the life of the church following his ascension to his Father. Certainly, the author does intend for us to read the discourse, which he presents as occuring before Jesus' death, as Jesus' message to his apostles and his church for the time following his resurrection and ascension.
The purpose of the last discourse of Jesus is to prepare his apostles and his church for our mission to be "sent...into the world" (17:18). Jesus explains that he is the true Israel who fulfilled the vocation that Israel was sent to perform, and that, as his disciples, we are incorporated into Israel to accomplish its mission in the world. Invoking the prophets' description of Israel as the Lord's vine (Isaiah 5:1-7; Jeremiah 2:21; Ezekiel 19:10-14), Jesus tells us, "I am the vine, and you are the branches" (15:5). As Israel had been chosen by the Lord among all the peoples, so now Jesus has choen his apostles and us to go bear fruit (15:16) to make known "the only true God" (17:3). Repeatedly Jesus promises us that he will do whatever we ask in his name (14:14; 15:16; 16:23-24). He warns us of persecution when we give testimony to him (15:18-16:4; 16:20-24), for "servants are not greater than their master" (15:20) who was hated by some.
Jesus assures us that, when he has ascended to his Father, the Spirit will come in his place to be with us (14:16-17; 14:26; 16:7). We are to trust in the illumination and guidance of the Holy Spirit to remember Jesus and to grasp the full import of his meaning.
He offers to us words of encouragement. "Do not let your hearts be troubled" (14:1). "Peace I leave with you" (14:27). "Abide in my love" (15:10). "Your hearts will rejoice" (16:22).
Finally, Jesus prays to his Father that we may be protected from evil while we carry out our mission in the world (17:15). He prays that we may be "one" in an unity of love so that "the world may believe" (17:21).
We cannot know if this great discourse in John 14-17 does come from the post-resurrection period when the risen Jesus appeared to his apostles to commission them to inaugurate the mission of the church in world history. It does seem to me to be a useful thought-experiment to read this passage as a post-resurrection experience of the apostles. Regardless of its historical location in the experience of the apostles, it is the word of the Lord to us to be his witnesses to the world by the power of the Holy Spirit.