ANSWER: The passage is designedly obscure. It may mean it is none of Peter's business if Christ should let John live on the earth till Christ should come to judge the world and wind up its history. This erroneous interpretation, "went forth among the brethren." I prefer to understand the words, "till I come," to mean the coming of Christ in the destruction of Jerusalem in A. D. 70, at least twenty years before John's death (Matt. 24:80-34; 16:28; 10:28). But some writers think this "coming of Christ" was his special manifestation of himself to John in Rev. 1:12-20.
— Steele's Answers p. 163.