It is likely that John the Elder wrote the Gospel of John, the three letters of John, and Revelation.
Gospel of John
The Gospel of John is unlike the three other Gospels. It can be considered to be a personal reflection on Jesus’ life and teaching by someone who knew him. Yet the work gives us no indication that its author was one of the twelve.
Whereas Jesus’ teachings in the Synoptic Gospels is via parables, aphorisms and in summaries such as in the Sermon on the Mount; it is also likely that his teaching method would oftentimes have been closer to the longer dialogues found in the Gospel of John. Indeed, if the entirety of the Sermon on the Mount were presented using the form of extended dialogues found in the Gospel of John, the size of such a work would have exceeded all reasonable bounds for a work produced at that time.
The impression we gain from the Gospel of John is that its author belonged to the elite of Jewish society (even if he would have been too young at the time to have played a prominent role during Jesus’ lifetime); he had a personal connection to the high priest (John 18:15). His grasp of high philosophy is quite apparent in the prologue found in the first chapter of this work. Throughout this work the author shows confidence in his handling of subtle concepts. For example, it is not surprising that he is the only one who records Jesus’ discussion with Nicodemus, a member of the Jewish Sanhedrin. On these grounds alone, it would be counter intuitive to assert that the author of this work was the fisherman from Galilee whose brother was James.
Letters of John
Who cannot be impressed with the similarity between the elevated prologue to the Gospel of John and the beginning of the 1 John. It is not a matter of simple borrowing, but rather of sharing the same thought world.
The prima facie case is that they share the same author, and this can only be undermined at this distance in time by arguments based on a difference in the ideas expressed or a difference found in literary forms of the two works. Yet, rather than showing a difference in authorship, they point towards a common authorship.
It has also been argued that similarity in style and content strongly point towards common authorship of all three letters, even though the second and third letters are really too short to make the case definitively.
The work is a prophecy self-attributed to John, a servant of Jesus Christ.
On literary style, there is a difference between this work and the other three works attributed here to John the Elder. On the other hand there is nothing in this work that cause an objective reader to say, “This is not from the same author.” The confidence of the author, his control of his material, and few verbal similarities, e.g. “bearing witness” in Rev. 1:2, the Logos motif found in Rev. 19:13.
Since there is no other plausible candidate, and there is no reason to doubt the authorship of this work by John the Elder, it is natural to associate this work with him.