Acts 20:29-31

I know that after I am gone, [false teachers like] ferocious wolves will come in among you, not sparing the flock; even from among your own selves men will arise, speaking perverse and distorted things, to draw away the disciples after themselves [as their followers]. Therefore be continually alert, remembering that for three years, night or day, I did not stop admonishing and advising each one [of you] with tears.