“Then came the disciples to Jesus apart, and said, Why could not we cast him out?”
King James Version (KJV)
17:17 O unbelieving and perverse generation - Our Lord speaks principally this to his disciples. How long shall I be with you? - Before you steadfastly believe?
17:20 Because of your unbelief - Because in this particular they had not faith. If ye have faith as a grain of mustard seed - That is, the least measure of it. But it is certain, the faith which is here spoken of does not always imply saving faith. Many have had it who thereby cast out devils, and yet will at last have their portion with them. It is only a supernatural persuasion given a man, that God will work thus by him at that hour. Now, though I have all this faith so as to remove mountains, yet if I have not the faith which worketh by love, I am nothing. To remove mountains was a proverbial phrase among the Jews, and is still retained in their writings, to express a thing which is very difficult, and to appearance impossible. #Matt 21:21|; Luke 17:6.
17:21 This kind of devils - goeth not out but by prayer and fasting - What a testimony is here of the efficacy of fasting, when added to fervent prayer! Some kinds of devils the apostles had cast out before this, without fasting.
Mt 17:19 Why could not we cast him out? The answer is, Lack of faith.