When the disciples went to Jesus askingwhy they couldn'tcast the demon out of the man, Jesus told them "This kind goeth not out but by prayer and fasting." (Matthew 17:21) There it is, plain and simple. Jesus, Himself, gave us simple instructions on how to cast out demons. Prayer and fasting is the key. Fasting is essential to the life of the Believer, if he/she wants to walk in power and experience victory.
Fasting is a way to honor God. By afflicting our flesh, we honor God by putting the Spirit above the flesh. We also honor God through fasting by fellowshiping with Him in suffering, although any suffering we may ever experience will never compare to that which Jesus suffered. Still, we have the opportunity to honor God in this way.
When we fast, we actually "kill" or put to death those fleshly desires that hinder us from serving God fully. When someone sgtruggles with lust, the best way to bring that fleshly desire under subjection of the Spirit is through fasting. This lets the flesh know Who rules or Who the boss is. Often when these desires get out of control, fasting is the number one way to get them under control. If you feed the flesh, the flesh will rule, but if you feed the Spirit, it will rule.
Fasting is a big part of spiritual warfare, although it is rarely taught on today. Believers must never forget that we are at war with our enemy, the devil. There is a continual battle going on between our flesh between spirit for the right to rule and reign. In order to win the war and walk in victory, we must fast and pray.
A person who spends their time watching television, listening to ungodly music, over eating, entertaining themselves with video games, etc. will reap the fruit of the flesh. While there is nothing wrong with watching television or playing video games, these things do not produce fruit of righteousness, nor do they edify the spirit or glorify God. On the other hand, if a person spend his/her time, meditating on the word of God, praying and/or listening to good preaching, that person will grow, developing in the things and gifts of God. They will become formidable foes to the enemy because they have been strengthened through the word of God. They have fortified themselves against the onslaught of the devil. A Christian who does not fast and pray will be weak and unable to cast out a demon.
Fasting is essential to walking in the power of God. If you want to walk at a high level in God, you must learn to fast and pray. Ask God to teach you how to fast and He will. Then ask Him to tell you when to fast and He will. The Holy Spirit will lead, teach, and guide you in the way you should go. He will show everything you need to know and do.