Summary: Pastors are to fulfil the ministry God has assigned with the help of strong, godly leaders from within the congregation.
“Pay careful attention to yourselves and to all the flock, in which the Holy Spirit has made you overseers, to care for the church of God, which he obtained with his own blood. I know that after my departure fierce wolves will come in among you, not sparing the flock; and from among your own selves will arise men speaking twisted things, to draw away the disciples after them. Therefore be alert, remembering that for three years I did not cease night or day to admonish every one with tears. And now I commend you to God and to the word of his grace, which is able to build you up and to give you the inheritance among all those who are sanctified.”  
Whether they accept it or not, leaders within a church are assigned responsibility that is not often spelled out in the bylaws of any congregation I have ever served. Whether deacons or elders, or whether they are simply looked up to as mature believers, those who provide leadership within a congregation bear an awesome responsibility before the Lord. The duty in view is as necessary as any given them in scripture or imposed by the membership—Be on a constant lookout for trouble and troublemakers within the congregation—watch for devils in the pew. A writer, from whom I purloined the idea for this particular message, refers to the devil in the pew as a DIP.  It is an appropriate designation as the insidious individual is prepared even to use violence to achieve his, or her, desires. Theirs is an all-out assault against righteousness, usually focused on removing God’s appointed overseer.
When he met them at Miletus, the Apostle Paul told the Ephesian leaders that they could expect deadly threats to the survival of the congregation; and these threats would arise from two sources—without and within [see ACTS 20:29 ff.]. The former they would have naturally anticipated; it is no secret that the evil one wants to destroy the churches, neutralising their effectiveness. Moreover, it is a given that Satan will use any means necessary to accomplish that nefarious goal. It was the latter source of danger—enemies arising from within the assembly itself—that must have surely have surprised the elders. Had those leaders been as trusting and naive as many of us within contemporary churches, they would have expected all the worshipers to be loving and gracious, faithful and trusting, and would have been blindsided by tyrants arising from their own number. So, Scripture warns us to be alert, to be watchful in both directions.
Keeping watch against such danger is not the job of the pastor alone. Granted, he is charged with this responsibility; but in a congregation of hundreds or even thousands, he needs eyes and ears other than his own. This multiplication of watching must begin even when the congregation is small. The pastor needs the deacons and Sunday School teachers—note that these individuals must be godly and mature—to keep their eyes and ears open, to remain ever vigilant. Plainly put, they are to watch out for the devils in the midst of the congregation.