Their teaching is false and can be recognized as such by their rejection of the authority Jesus gave his apostles to tell his story. These are the apostles. He is the last one living, but he still thinks of himself as part of a community of men who were given responsibility from God to write the New Testament, to accurately teach the truth about Jesus. If any false teacher contradicts what the apostles taught, we are not to listen to them. We don't need to be afraid when confronted by false religion any more than we do when our hearts condemn us.
False teachers come and go.
I have lived long enough to have seen many bad versions of Christianity fade away. They aren't true, and we don't need to be afraid of them. Greater is the one who is in us than the one who is in the world. Thank God for these reminders that it is God's work, not ours, to speak to our hearts and to deal with falsehood in the world. With these reassurances, we can go on and learn the lessons of love.