Christians! Is he talking about other religions?

I see no mention of any other religion here. John is writing to the churches around Ephesus, a letter to be circulated among them. The word "antichrist:" is mentioned three times here, and again in 2 John 7. It occurs nowhere else in the Bible, not even in Revelation. John's language implies that his readers had been taught to expect an anti-Christ in connection with the closing days of the Christian era (2:18). But John applies the word not to one person but to a whole group of anit-Christian teachers who represented themselves as Christian but spoke a heretical message).