CLAIM: John concludes his gospel by referring to himself as the singular author (“This is the disciple who is testifying…”). However, he also uses the first person plural (“We know that his testimony is true”). Does this mean that John’s disciples wrote this book?
RESPONSE: John doesn’t write that many people wrote this book. He explicitly writes in the first person singular (“This is the disciple”), the present tense (“This is the disciple”), and attributes authorship to himself (“wrote these things”). By contrast, he writes that others “know that his testimony is true.” According to these others (whomever they might be), they are not coauthors according to the text. Rather, they are people who affirm the veracity of John’s gospel. This is similar to what John wrote earlier, affirming his own work (Jn. 19:35).
However, we reject the idea that the entire gospel was written by John’s later disciples. If this theory was true, then why wouldn’t the Johannine School name John (their founder) as the author? As Morris argues, “It is nothing short of astounding that those who argue for this ‘school’ behind our Fourth Gospel take so calmly their idea that neither the original author of the Gospel, nor any of its various redactors, ever mentions the founder of the ‘school’ when they are narrating the very events that brought the ‘school’ into existence. What sort of ‘school’ is this?”
Consequently, we see that there are two good options for understanding this text:
OPTION #1: John is using the editorial “we”
Carson is inclined to believe that this is an editorial “we,” which John is prone to use throughout his writing (e.g. Jn. 3:11; 1 Jn. 1:4; 3 Jn. 12). In verse 25, John returns to the first person singular (“I suppose…”).
OPTION #2: This one verse was added by the disciples of John
Morris, Kruse, Whitacre, and Tenney believe that an early church wrote this clause (“we know that his testimony is true”) and added it to John’s writing to show that they affirmed what was written. This is much different from saying that the disciples of John wrote the entire gospel (!). Instead, this is an affirmation of John’s gospel from his closest disciples and earliest readers. Virtually every manuscript of John contains this verse, so this must have been added very early.
 Leon Morris, The Gospel According to John (Grand Rapids, MI: Wm. B. Eerdmans Publishing Co., 1995), p.22.
 Carson, D. A. (1991). The Gospel according to John (p. 684). Leicester, England; Grand Rapids, MI: Inter-Varsity Press; W.B. Eerdmans.
 Morris, L. (1995). The Gospel according to John (p. 777). Grand Rapids, MI: Wm. B. Eerdmans Publishing Co.
 Kruse, C. G. (2003). John: an introduction and commentary (Vol. 4, p. 389). Downers Grove, IL: InterVarsity Press.
 Whitacre, R. A. (1999). John (Vol. 4, p. 500). Downers Grove, IL: InterVarsity Press.
 Tenney, M. C. (1981). John. In F. E. Gaebelein (Ed.), The Expositor’s Bible Commentary: John and Acts (Vol. 9, p. 203). Grand Rapids, MI: Zondervan Publishing House.