New Defender's Study Bible Notes
17:19 Why could not we. The disciples had previously been empowered to cast out evil spirits from demon-possessed people (Matthew 10:1,8), and presumably had done so. It may be, however, that they had begun to have faith in their own power, rather than in God. If one’s faith is in the omnipotent Creator, nothing is impossible (Matthew 17:20), for He controls all things. However, such true faith will also recognize that the prayer of faith will acknowledge dependence on the will of God (Luke 22:42; I John 5:14-15).