CLAIM: Critics argue that this chapter was a later interpolation onto John’s gospel for several reasons: First, the gospel of John appears to conclude with John’s purpose statement of the book (Jn. 20:30-31). Second, critics theorize that later authors (the Johannine community?) had to correct an error that Jesus would return before John’s death. Since John died, it must therefore be an interpolation by a later author or authors. Is this the case?
RESPONSE: We reject this view for a number of reasons:
First, there is no existing Greek manuscript that excludes John 21. The closest one finds is a late 5th or 6th century Syriac manuscript that lacks John 21. However, this can hardly be considered good evidence.
Second, the final chapter is similar in style, vocabulary, and grammar. Plummer sees at least 25 examples of similarity between John 21 and the rest of the gospel.
(1) The use of the term “sheep” (Jn. 21:16-17; Jn. 10).
(2) The language of indicating how Peter would die (Jn. 21:19; 12:33).
(3) “Truly, truly…” (Jn. 21:18; 1:51; 3:3; etc.).
Bultmann notes 28 linguistic differences between John 21 and the rest of the gospel. However, Carson notes, “Most of these are so tied to the subject matter that they cannot be viewed as particularly significant: e.g. the Greek words for ‘fish’, ‘net’, ‘feed’, ‘take care of [my sheep]’, ‘he wrapped’, ‘naked’ and the like.” Moreover, we reject word-counting as powerful evidence. After all, Carson writes, “there is reason to suspect that if similar tests were applied to several other chapters in John, the results would be no less ambivalent.”
Doesn’t John 20:30-31 seem like the end of the gospel?
If John ended with chapter 20, the reader would be left wondering, “Whatever happened to Peter?” Not only did John want to tie up the loose end of Peter’s restoration, but he also wanted to explain the death of Peter in light of Jesus’ teaching on suffering and persecution (Jn. 15:18-16:4).
Moreover, we cannot assume what this author would or would not consider an apt conclusion to his book. After all, 1 John 5:13 seems like the end of John’s epistle. However, John had more to write.
Finally, notice the entire literary composition of John. The beginning of the gospel has a prologue (Jn. 1:1-18), and the end has an epilogue (Jn. 21:1-25). Indeed, Bauckham notes, “Just as the Prologue goes back in time to creation, so the Epilogue previews the future mission of the disciples, symbolized by the miraculous catch of fish, and focuses especially on the different roles that Peter and the Beloved Disciple are to play in it. The time projected by the Epilogue runs to the parousia (future coming) of Jesus. Its last words, in [Jn. 21:23], are Jesus’ words, ‘until I come.’” This acts as an inclusio (or “bookend”) to the opening: “In the beginning” (Jn. 1:1).
 See footnote. Morris, L. (1995). The Gospel according to John (p. 757). Grand Rapids, MI: Wm. B. Eerdmans Publishing Co.
 A. Plummer, The Gospel according to S. John, Cambridge Greek Testament (Cambridge, 1882), 348-357. Cited in Morris, L. (1995). The Gospel according to John. Grand Rapids, MI: Wm. B. Eerdmans Publishing Co.
 R. Bultmann, The Gospel of John: A Commentary, translated by G. R. Beasley-Murray, R. W. N. Hoare, and J. K. Riches (Blackwell, 1971), 700-701. Cited in Carson.
 Carson, D. A. (1991). The Gospel according to John (pp. 664-665). Leicester, England; Grand Rapids, MI: Inter-Varsity Press; W.B. Eerdmans.
 Carson, D. A. (1991). The Gospel according to John (p. 666). Leicester, England; Grand Rapids, MI: Inter-Varsity Press; W.B. Eerdmans.
 Richard Bauckham, Jesus and the Eyewitnesses (2nd edition, Grand Rapids, MI: Wm. B. Eerdmans Publishing Co. 2017), p.364.