Little children, these are the end times, and as you heard that the Antichrist is coming, even now many antichrists have arisen. By this we know that it is the end times.
As ye have heard, antichrist cometh ...  Although only the singular is used here, it is clear from what John at once wrote that there were many of these.  Where had the Christians heard of this?  From the teachings of Christ, as recorded in  Matthew 24.