It is widely acknowledged that John 20 stands as an appropriate ending to the book. It presents us with, what Beasely-Murray calls, a "total picture of the Easter story": the empty tomb, the witness of Mary Magdalene, the confirmation of the empty tomb by two disciples, an appearance of Jesus to Mary and other disciples, the reception of the Spirit and Jesus' commissioning of the apostles.
Moreover, the chapter ends with an epilogue, which seems to bring the book to a close: "Now Jesus did many other signs in the presence of the disciples, which are not written in this book; 31 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" (John 20:30-31).
Beasely-Murray thus concludes: "Had he planned to record the appearance(s) to Peter and his colleagues narrated in chap. 21 he would have composed chap. 20 differently" [George R. Beasely-Murray, John (2nd ed.; WBC 36; Columbia: Thomas Nelson, 1999), 395].
Looking at chapter 21, many scholars argue that it was written by another hand. One of the reasons for this is that it seems as though chapter 21 does not follow neatly from chapter 20. In addition to the fact that the 20:30-31 seems to tie up the Gospel narrative in such a way as to conclude the Gospel, some have argued that chapter 21 also seems detached from what has come before it. John Breck lays out the most common reasons given in support of this line of thought:
1. The epilogue in chapter 20:30-31 serves as an apparent conclusion, as mentioned above.
2. Chapter 21 does not show us the disciples setting out on the mission given to them by Jesus in chapter 20--rather than going out to evangelize, the apostles go fishing.
3. Whereas chapter 20 called for believing without seeing, chapter 21 seems to emphasize the importance of seeing and believing.
4. There is a reference to "we" in John 21:24, which most see as an indication of later redactional work.
5. Chapter 21 seems to focus on concerns of the Church--addressing issues of the later Christian community.
6. Some of the themes developed in chapter 21 are only found in places in the Gospel where scholars believe later interpolations have been introduced.
7. Chapter 21 contains language and stylistic elements not found elsewhere in the Gospel.
[These arguments are laid out by John Breck, "John 21: Appendix, Epilogue or Conclusion?, in St. Vladimir's Theological Quarterly 36 (1992): 27-49].
Here I want to look at these seven arguments.
1. Double endings seem to be characteristic of Johannine literature. Breck cites the work of Peter Ellis, who has noted the use of double-endings in Johannine literature. Consider John 12:36b-37: “When Jesus had said this, he departed and hid himself from them. 37 Though he had done so many signs before them, yet they did not believe in him.” Interestingly, the passage continues on to say that the disbelief of the Jews fulfilled a prophecy of Isaiah (12:38-43). It then relates a final speech of Jesus (12:44-50). This especially curious since John has already stated that Jesus went and hid himself from the people!
One should also consider the close of 1 John 5. 1 John exhibits numerous similarities to the Fourth Gospel. There seems to be a deliberate imitation of the Fourth Gospel in the stylized prologue (cf. John 1:1-18; 1 John 1:1-4). Both seem to share a closing statement of purpose (cf. John 20:31; 1 John 5:13). Moreover, after the summary statement of 1 John 5:13, the letter continues on—paralleling what is found in the fourth Gospel. Breck concludes: “John 20:30ff., like 1 John 5:13, thus seems to represent a stylistic device that serves as a pivotal ‘definition of purpose’ between the main body of the writing and the conclusion.”
Moreover, John 20:35ff. and 21:25 seem to form a kind of inclusion.
A. (20:30f). Inclusion: Many signs.
B. 21:1-14. The Beloved Disciple and Peter: the Beloved Disciple recognizes Jesus
C. 21:15-19a: Peter’s rehabilitation
B.’ 21:19b-24: The Beloved Disciple and Peter: the Beloved Disciple (as the author) witnesses to Jesus
A.’ (21:25) Conclusion: the overwhelming number of signs of Jesus.
In fact, the Fourth Gospel frequently makes use of this kind of chiasmus structure. For example, in John 19:19-22 we see the following:
A. (19:19) Pilate also wrote a title and put it on the cross; it read, “Jesus of
Nazareth, King of the Jews”
B. (19:20a) Many of the Jews read this title, for the place where Jesus was crucified near the city
C. (19:20b) and it was written in Hebrew, Latin and in Greek.
B.’ (19:21a) And the chief priests of the Jews then said to Pilate,
A.’ (19:21b) “Do not write, ‘The King of the Jews,’ but ‘This man said, I am King of the Jews.’” Pilate answered, “What I have written I have written.”
It should also be pointed out that John 21 forms a kind of inclusion with the opening chapter of John. Here once again Jesus is found on the shores of Galilee with the fishermen. Indeed, as Ellis has shown, there are a number of parallels between 1:19-51, 2:1 and chapter 21, such as the repetition of names, terms and expressions (Simon, son of John; Jesus, Son of God; Nathanael; two unnamed disciples; Cana; Galilee; “follow me”; “who are you?”; “bear witness”; “turned and saw following”; “remain”; “word”, etc.). It is also noteworthy that chapter 21 relates that the beloved disciple is he that lay at Jesus bosom (kolpos) at the Last Supper (21:20). This echoes John 1:18, which tells us that Jesus came from the bosom (koplos) of the Father. Ellis concludes, “As Jesus is to the Father so the Beloved Disciple is to Jesus!”
2. If one takes seriously the Lukan account of Pentecost then one of course would not yet expect the apostles to go out yet to evangelize. Yet, the mission given to the disciples in the previous chapter is probably not completely forgotten, since the catching of fish certainly seems an appropriate metaphor for their future ministry.
3. In chapter 21, it is only the Beloved Disciple who recognizes Jesus and only after Jesus’ voice was heard and obeyed. I would agree with Blomberg, who argues that objections (3) and (5) are “proabaly false disjunctions.
4. The use of “we” may not necessarily indicate a later redaction. Breck describes the peculiar Johannine tendency to switch from the singular to the plural. One might look at Jesus’ own words in 3:11-13, in which he speaks of himself in the singular, plural and third persons within the space of three verses:
“Truly, truly, I say to you, we speak of what we know, and bear witness to what we have seen; but you do not receive our testimony. 12 If I have told you earthly things and you do not believe, how can you believe if I tell you heavenly things? 13 No one has ascended into heaven but he who descended from heaven, the Son of man.”
In some cases this involves the inclusion of the witness of the community of faith with the author. Such is likely the case in John 1:14, where the author says, “we have beheld his glory.”
Here I will cite Breck’s comments in full:
“In John 21:24 the author identifies the Beloved Disciple as the primary witness to the events recounted―not just in the resurrection scene of this chapter but in the entire Gospel. His words reaffirm what was already claimed in 19:35, that the witness he bears is true and dependable. It is significant that in 19:35, as in 21:24a, the verb is in the present tense: he has borne witness yet “he knows that he tells (λέγει) the truth”; “this is the disciple who is bearing witness (ὁ μαρτυρῶν) to these things.” The author of the Gospel, in ch. 19 as in ch. 21, declares that he witnessed both Jesus’ death and resurrection, and that the written testimony comes from his hand… In any event the principle of affirming eye-witness tradition is consistent throughout the Gospel, from the first chapter to the last (1:14; 3:11; 19:35; 21:24). The statements in 21:24, then, simply because they reflect the collective 'we' of the community’s faith and witness, cannot be used to argue against the authenticity of this passage.”
We should also note here verse 23: “The saying spread abroad among the brethren that this disciple was not to die; yet Jesus did not say to him that he was not to die, but, ‘If it is my will that he remain until I come, what is that to you?’” While some have seen this as an addition made after the death of the Beloved Disciple, nothing in the text necessarily leads to that conclusion.
5. While it is true that the end of chapter 21 focuses on ecclesiological issues, one need be careful not to overstate this dimension of the chapter. The first part is clearly focused on a resurrection appearance of Christ―with many similarities to chapter 20, it should be noted. It should also be mentioned that concern for the community of faith is not at all foreign to the Gospel. One need only think of the Farewell Discourse in chapters 13-16 (the promise of the sending of the Spirit upon the disciples; the warning of future persectutions; the vine and branch imagery used for Christ and believers; Jesus’ insistence that the disciples have been “chosen”; the notion of disciples “bearing fruit”). In fact, the failed fishing expedition seems to corroborate Jesus’ warning in chapter 15:5: “apart from me, you can do nothing.” The final prayer of Christ in chapter 17 is also clearly concerned with the fate of the Christian community.
6. The supposed interpolations linked to John 21 are actually probably authentic. These interpolations are 6:51c-58 and 5:28-29. A close examination reveals their connection to chiastic structures within the text.
7. In regard to the language differences, much is due to the unique content of chapter 21. In addition, see the work of de Solages and Vacherot who have offered a detailed examination, showing that this chapter is not on the whole significantly more different than other sections within the Gospel, which contain unique elements.
I must admit that I have not read everything out there on this issue. I have to do a lot more work before I can insist that John 21 is clearly authentic. However, very few of those who dismiss the authenticity of chapter 21 seriously engage with the arguments above. That concerns me a great deal.
My conclusion--at this point--would be that given what we have seen above, I think it is difficult to simply shrug off the authenticity the chapter. In fact, a strong case can be made in favor of seeing it as part of the literary whole of the Gospel. Merely appealing to the "majority opinion" among scholars will not do. We might also add one observation that we have not mentioned: there is no manuscript evidence to indicate that either the title of the Gospel or last chapter were ever added later to the document. Such conclusions are not based on what is found in ancient manuscripts but rather on certain hypothetical conclusions.
Furthermore, I should add that it seems to me that the presence of the prologue at the start of the Gospel makes it likely that the Gospel would conclude with a kind of epilogue. Of course, that in-and-of-itself does not constitute a very strong argument. However, in my opinion, taken with what we've seen above such a view garners some force.
 Peter Ellis, “The Authenticity of John 21,” in St. Vladimir’s Theological Quarterly 36 (1992): 17-25; idem., The Genius of John: A Composition-Critical Commentary on the Fourth Gospel (Collegeville: Liturgical Press, 1984), 290-312.
 Breck, “John 21,” 29.
 For a fuller treatment see Peter Ellis, “Inclusion, Chiasm and the Division of the Fourth Gospel,” in St. Vladimir’s Theological Quarterly 47 (2003): 131-154.
 Peter Ellis, “The Authenticity of John 21,” 24.
 Craig Blomberg, The Historical Reliability of John’s Gospel (Downers Grove: InterVarsity Press, 2001), 272.
 Breck, “John 21,” 33.
 For a detailed look, see Breck, “John 21,” 35; Ellis, The Genius of John, 90ff.
 Bruno de Solages and J. –M. Vacherot, “Le chapitre XXI de Jean: est-il de la meme plume que le reste de l’Evangile?" in Bulletin de littérature ecclésiastique 80: 96-101.