The pharisee 's here knew what Jesus was saying here when he used this phrase "by whom do your sons cast them out?"It was common pratice in that day to exercise , or at least try to exercise demons from people. The whole Idea here ,is I believe , spiritual warfare from the advantage point of The Kingdom of God. Here's what I mean. The victory is already won , the promised land has already been given to Israel, However - that doesn't relieve us of our duty to self deniel in prayer and intercession to see the full manifestation of that victory, in the same way that Israel had to go in and possess the land. Through pray with fasting we go in and possess the land or the release of those held captive by satan. One might ask -- what about all the missionaries who have been faithful and have seen no fruit or souls won to Jesus? Thier labors are not invain, as the scriptures say , but with the eyes of faith they embrace the reward and they will see in heaven the results of thier obedient sacrifices!In the same way we will also if we faint not. We walk by faith not by sight.