Acts 20:28-31

Take care and be on guard for yourselves and for the whole flock over which the Holy Spirit has appointed you as overseers, to shepherd (tend, feed, guide) the church of God which He bought with His own blood. 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.