Mark 9:28,29

Why couldn't we drive the spirit out? Jesus said: "It was because you did not have enough faith" (Matthew 17:20). No demon could defy them if they had faith [on faith, see note on James 2:19-20]. Only prayer can drive this kind out. Faith reaches out through prayer to seize the power of Christ [for one who is already a Christian, John 9:31]. [Some versions add: "and fasting" from Matthew's Gospel. Do not think of prayer and fasting as a ritual of exorcism. These are general principles of spiritual growth.] When faith focuses on God's act in Christ, we forget self and can do many things through Christ. When faith turns inward to "self," we cannot drive out the "evil spirits" of worldliness, selfishness, greed, immorality, etc. See Peter's example Matthew 14:29-30.