I would guess that ISIS is probably converting more people to Christianity across the world because of their evil nature and heinous acts than all the missionaries combined. People witness their brutality and destruction and they know deep inside that this behavior does not come from a good and righteous god. In the end they will cause their own destruction and all of those who support them knowing full well who and what they stand for.
God works in many ways. He uses as many means to reach the lost as there are lost people, because He meets each unique individual directly. He is worthy of all praise.