Many have observed this is a strange ending for the letter, there is nothing about idolatry in the entire book so it seems odd for John to drop this line as a conclusion to the letter. Was there another paragraph which explained this line? Did the usual ending of an epistle get lost before the letter was added to the canon?
That this is the seventh time John has called his readers children is significant. This is a planned statement not a last minute addition to fill out a page of papyri. In fact, Jobes says the original readers would have read this like a punchline, the rhetorical conclusion to the letter, answering the question the whole letter has been asking, if all this is true and we know what is real, can we really worship idols? (Jobes, 1, 2, & 3 John, 242).
For many interpreters, this is a command to avoid literal idolatry. Rejection of the Roman imperial cult led to some kind of persecution of John’s community. “Those who deny that Jesus is the Christ in this letter (2:22–23; 4:3; cf. 2 John 7) are people who yield to these pagan demands so as to avoid martyrdom” (Kruse, Epistles of John, 200).
For others, this command is a metaphor. Taking the word idol as “phantom,” Sugit argued this is a warning away from Docetism (J. N. Sugit, “I John 5:21,” JTS 36 (1985): 386–90). Raymond Brown took idolatry as a reference to the successionists, stay away from the liars and deceivers who John has called the antichrists (Epistles of John, 627–28).
“By idols he means not only images of the gods, but all false or counterfeit notions of God such as lead to the perversions of religion against which he has written” (Dodd, Johannine Epistles, 141)
“Most modern interpreters identify ‘idols’ with the idolatry of the secessionists who left the worship of the true God to follow after a false Christology” (C. Marvin Pate, The Writings of John, 316)
But idolatry is a strange metaphor for some kind of Christological error. The people who read this letter originally lived a world full of idolatry, it was impossible to avoid gods living in first century Ephesus!
Although this is necessarily speculative, I suggest the people who left John’s churches and claim not to sin offered worship to Rome or Artemis in order to avoid persecution. Several times in the letter John has implied that the opponents think that they do not sin. There’s no reason to think that sin would not include idolatry, so that worship of the Roman Empire in order to appear to be a good and loyal citizen would be perfectly acceptable to them. As I suggested in a previous post, few modern Christians consider pledging allegiance to the American flag to be idolatry. But for a person living in first century Ephesus, to “pledge loyalty” to the Roman Empire meant some kind of participation in the Imperial Cult.
Once again we find ourselves in the same territory as the seven letters of the Book of Revelation. There are several references there to people who participate in some form of idol worship, and certainly the rest of the book of revelation is about the worship of the beast, if the beast is Rome then the Roman imperial cult is not far from the surface.