John Calvin Commentary Acts 20:29

"I know that after my departing grievous wolves shall enter in among you, not sparing the flock;" — Acts 20:29 (ASV)

For I know. Paul now exhorts the Ephesians to watch diligently because of the necessity, which is a most sharp spur. For he says that “grievous wolves are ready to invade the sheepfold.” This is a reality always facing the Church: to be hated by wolves. Therefore, there is no time to sleep.

But the more numerous and the more hurtful those are who break in, the more watchful the pastors must be. For God sometimes grants some relief from trouble, so that the flock may be fed quietly and peaceably. And just as when the weather is fair and clear, the sheep are fed more safely in the fields, and there is more danger when the air is cloudy and dark, so the Church of God sometimes has some fair weather granted to it.

After that comes a troublesome time, which is more suitable for the subtlety of wolves. Therefore, Paul means that it is incumbent upon them to be more watchful than they have been until now, because greater dangers hang over their heads.

But the question is, how did Paul know this? First, we need not doubt that his presence was a powerful force in driving away wolves. And it is no wonder if the power of the Spirit, which shines in the ministers of Christ, bridles the wicked so that they dare not utter their poison; indeed, if that heavenly light drives away much of Satan’s darkness.

Therefore, since Paul knew that the malice of Satan was suppressed for a time by his diligent efforts, he easily foretold what would happen after his departure. It is also likely that he was informed by the Lord through the spirit of prophecy, so that others might be admonished by him, as we see indeed happened.

However it may be, whenever faithful pastors depart, let us learn that we must beware of wolves, whom they can hardly drive from the sheepfolds, though they watch with utmost vigilance.