IN THOSE DAYS, a certain man came to Jesus, fell down before Him, and said, “Teacher, I brought You my son who has a spirit that makes him unable to speak. Whenever the spirit seizes him, it throws him to the ground, and my son foams at the mouth and grinds his teeth and becomes rigid. I asked Your disciples to cast it out, but they were not able.” And Jesus answered, “You faithless generation, how much longer must I be with you? How much longer must I put up with you? Bring him to Me.” And they brought the boy to Him, and when the spirit saw Jesus, immediately it convulsed the boy, and he fell to the ground and rolled about, foaming at the mouth. Jesus asked the father, “How long has this been happening to him?” The father said, “From childhood. It has often cast him into the fire and into the water, to destroy him. But if You are able to do anything, have mercy on us and help us.” Jesus said to him, “If you are able! All things are possible to the one who believes.”Immediately the father of the boy cried out, “I believe. Help my unbelief!” When Jesus saw that a crowd came running together, He rebuked the unclean spirit, saying to it, “You spirit that keeps this boy from speaking and hearing, I command you, come out of him, and never enter him again!” After crying out and convulsing the boy terribly, the spirit came out, and the boy became like a corpse, so that many said, “He is dead.” But Jesus took him by the hand and raised him up, and the boy stood up. When Jesus had entered the house, His disciples asked Him privately, “Why could we not cast the spirit out?” Jesus said to them, “This kind can come out only through prayer and fasting.” They went on from there and passed through Galilee, but Jesus did not want anyone to know it because He was teaching His disciples, saying to them, “The Son of Man is to be betrayed into the hands of men, and they will kill Him, and three days after being killed, He will rise again.”
Saint John Chrysostom explains how God heals us through prayer and fasting,
Do you see how He now proceeds to lay beforehand in them the foundation of His doctrine about fasting? You see, at any rate, how many blessings spring from both prayer and fasting. For he who is praying as he should, and fasting, does not have many wants, and he who does not have many wants can't be greedy. He who is not greedy, will also be more disposed for almsgiving. He who fasts is light, and winged, and prays with wakefulness, and quenches his wicked lusts, and propitiates God, and humbles his soul when lifted up. Therefore, even the apostles were almost always fasting. He who prays with fasting has his wings double, and lighter than the very winds, nothing is mightier than the man who prays sincerely. But if your body is too weak to fast continually, although you can't fast, yet you can avoid luxurious living.