It is almost always internal
28 Keep watch over yourselves and all the flock of which the Holy Spirit has made you overseers. Be shepherds of the church of God,[a] which he bought with his own blood.[b] 29 I know that after I leave, savage wolves will come in among you and will not spare the flock. 30 Even from your own number men will arise and distort the truth in order to draw away disciples after them.31 So be on your guard! Remember that for three years I never stopped warning each of you night and day with tears.
Let us take heed of Paul’s warning when it comes to attacks against the church – it will always almost be INTERNAL.
The most crucial person to guard against the church is YOU. Keep watch over yourself. If there is someone who could destroy the church – it is you. It could be our pride, lust or unresolved sin. Thus the warning to keep watch over ourselves.
Second, is the leadership team around you. The people you keep close and empower for leadership. I am not saying be paranoid but rather actively keep the unity of your leaders by building solid and strong relationships. Build real friendship based on the gospel and not based on work. Also make sure to build a culture of transparency and high trust. In this way, you will be able to sense savage wolves roaming around you.
Be pro-active when you see someone diverting away from the spirit of the gospel and the doctrines of the faith and confront lovingly because it always almost be internal.