Matthew 17:14–21 (NKJV, also appearing in Mark 9:14-29; Luke 9:37-43, 17:5-6)
14 And when they had come to the multitude, a man came to Him, kneeling down to Him and saying, 15 “Lord, have mercy on my son, for he is an epileptic and suffers severely; for he often falls into the fire and often into the water. 16 So I brought him to Your disciples, but they could not cure him.”
17 Then Jesus answered and said, “O faithless and perverse generation, how long shall I be with you? How long shall I bear with you? Bring him here to Me.” 18 And Jesus rebuked the demon, and it came out of him; and the child was cured from that very hour.
19 Then the disciples came to Jesus privately and said, “Why could we not cast it out?”
20 So Jesus said to them, “Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you. 21 However, this kind does not go out except by prayer and fasting.”
Jesus is approached by a caring and desperate father concerning his son. His son had epilepsy, called an unclean spirit in the King James Version, and suffered from seizures often. The disciples could do nothing for them, so the father came to Jesus. Jesus rebuked them for unbelief, and then cast out the demon, healing the child. The disciples questioned privately why they could not cast out the demon. Jesus again cited their unbelief, and gave them a prescription for greater faith and success with exorcism. He told them that some spirits only go out by prayer and fasting.
This discourse highlights the fact that spiritual authority is not to be taken for granted, but must be cultivated and maintained. The spiritual disciplines of prayer and fasting are essential for spiritual authority and power. No one should presume to be able to enter into spiritual warfare without spiritual preparation. Maintain your walk with God in order to maintain your spiritual sensitivity and faith. Then, you can cast out demons and the sick may be healed, and you can discern spiritual truth and right from wrong. Jesus taught His disciples much, but small lessons like this made them into apostles and powerful spiritual leaders. Do not neglect the spiritual disciplines, and do not neglect your relationship with the Lord. A strong relationship with Christ increases spiritual authority. He uses those whom He knows.
Artwork from https://www.oasischurchperth.com/prayer-and-fasting-guide-2018/