God works by man

All this is indeed the work of God. It is God alone who can cast out Satan. But he is generally pleased to do this by man, as an instrument in his hand, who is then said to cast out devils in his name, by his power and authority. And he sends whom he will send upon this great work, but usually such as man would never have thought of. For his ways are not as our ways, nor his thoughts as our thoughts (Isaiah 55:8). Accordingly he chooses the weak to confound the mighty, the foolish to confound the wise (1 Corinthians 1:27), for this plain reason: that he may secure the glory to himself, that no flesh may glory in his sight (1 Corinthians 1:29).