Expositor's Bible Commentary Commentary Mark 9:29

"And he said unto them, This kind can come out by nothing, save by prayer." — Mark 9:29 (ASV)

Mark answers why the nine disciples were powerless to act in behalf of this boy. In private they asked Jesus about their failure, especially since they had been given authority over evil spirits (6:7) and had successfully cast out many demons before this incident (6:13). Jesus’ answer suggests that they had taken for granted the power given them or had come to believe that it was inherent in themselves. So they no longer depended prayerfully on God for it, and their failure showed their lack of prayer.