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We don’t know that.
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Only in John is the “Beloved Disciple” mentioned per se. And, only in John is John never mention by name.
3 In all the Gospels it is Peter, James & John who are the favoured 3. Only those 3 got to witness the Transfiguration, etc. Same as at Gethsemane. And, we know it wasn’t Peter from various mentions of both in John, including Peter talking about the “Beloved disciple” and asking about him. Thus, indeed, the “Beloved Disciple” could possible James (Johns brother) but that James died very early, just after Jesus.
4 Now, it seems from internal evidence that John was written by the “beloved apostle” -or at least was made to appear that way. If either- it can’t have been James (everyone knew he died too early). Thus, the “beloved disciple” could only have been John. John lived long enough to have had a hand in the writing of his Gospel. John had his own group of followers at Ephesus, and internal writings and later Christian legend show that this Gospel likely came from there- or was part of their philosphy.
And, it was considered as fact by AD200.
Also note that it is just considered a “good guess” that in J9:22, 16.2 and 12.42 that what John is referring to is “Test Benediction” that was introduced by Rabbi Gamaliel II c85AD. Certainly the issue had come up earlier.
And now John is gaining far more support. I quote “The Oxford Comapnion to the Bible”: “Since roughly the middle of the 20th century support has been growing for the view that the basic tradition underlying John’s Gospel may be historically more reliable that previously acknowledged.” It goes on to show how the Quran scrolls, recent Archaeological discoveries and a new literary comparison all have turned contemporary thought around on the Gospel of John. John son of Zebedee is now widely considered to be (at least to some degree) the author of that Gospel. DtC is out of date on this.