For I know this, that after my departing shall grievous wolves enter in among you, not sparing the flock.
Note 9 at Ac 20:29: If Paul knew these terrible things were going to take place in the church at Ephesus after he left, why didn't he just rebuke the devil and stop those things from happening? Because he didn't have the authority to do that.
Some people have taken man's authority over the devil to the extent that they fault anyone who has problems. Paul would have certainly been guilty by that standard. The truth is that we have not been promised the absence of battles but the ability to win every battle. However, when other people's wills are involved, we cannot stop evil from coming their way. They have a choice.
Paul charged these elders to be faithful to feed the flock; that was their best protection against Satan's onslaught. This worked to such a large degree that some historical accounts record as many as 70,000 to 100,000 people in the church at Ephesus (see note 3 at Ac 18:19) during the time Timothy was bishop there (see note 1 at Ac 16:1).
However, the church at Ephesus didn't prosper just because Paul was the one who started it and Timothy was its first bishop; it was because the people believed for themselves what Paul and Timothy ministered. Within a few hundred years, the church at Ephesus became virtually extinct because God's Word ceased to be taught and believed.