From a Broken Heart and Contrite Spirit:
Remember His disciples who had been following Him, who were His faithful followers, those disciples couldn't fix this boy. Those disciples had given up everything to come and follow Him. They knew much more than this man. But their knowledge did them no good.
Jesus healed the boy. After the incident the disciples came to Him and said, "Why could we not cast him out?" Christ answered them and said, "This kind can come forth by nothing, but by prayer and fasting."
Why do you have to be afflicted by prayer and fasting, if you're a follower of the Lord, in order to get to the point you can accomplish this? Because you don't fall prostate, crying out with tears from a broken heart and a contrite spirit. If this man, in this condition, can say, "I believe, help thou mine unbelief." If this man can do this and have the Lord on his behalf work a miracle, you too can believe enough, you too can accomplish what you desire, you too can come to Him.
Matthew covers the same incident. This is Matthew chapter 17, beginning at verse 19: "Then came the disciples to Jesus apart, and said, Why could not we cast him out? And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you. Howbeit this kind goeth not out but by prayer and fasting."
Faith as a grain of mustard seed is what the Lord said they needed. The defect does not consist in the absence of faith in the Lord. The defect is the arrogance and hardness of the heart that prevents you from crying out, in the realistic and anguish of your heart, looking to God who is trying to bring you to Him. That depth of humility, that status of being someone who is utterly harmless, that condition in which you present no threat to the righteous, you are harmless as a dove, and you seek only the betterment of others. That is who God is, and what you must become in order for God to be able to redeem you to be like Him. That involves you voluntarily changing to be that person, by your submission to Him. Because there is no reason to give to the proud, the vain, and the warlike, the ability to torment and to afflict others because they have authority from God. There is every reason to give authority only to someone who would ultimately be willing to give the rain to fall on the righteous and the wicked, and to make the sun shine on both the righteous and the wicked. They can be trusted with the power of God, because the power of godliness consists in this kind of a heart. And in this kind of heart God, can accomplish anything.