If the solemnities of our fasting, though frequent, long, and severe, do not serve to put an edge upon devout affections, to quicken prayer, to increase Godly sorrow, and to alter the temper of our minds, and the course of our lives, for the better, they do not at all answer the intention, and God will not accept them as performed to Him.
Author: Matthew Henry Source: A Commentary on the Whole Bible, Funk and Wagnalls, v. 4, p. 1478.
Fasting in the biblical sense is choosing not to partake of food because your spiritual hunger is so deep, you determination in intercession so intense, or your spiritual warfare so demanding that you have temporarily set aside even fleshly needs to give yourself to prayer and meditation.
Fasting, if we conceive of it truly, must not be confined to the question of food and drink; fasting should really be made to include abstinence from anything which is legitimate in and of itself for the sake of some special spiritual purpose. There are many bodily functions which are right and normal and perfectly legitimate, but which for special peculiar reasons in certain circumstances should be controlled. That is fasting.
The purpose of fasting is to loosen to some degree the ties which bind us to the world of material things and our surroundings as a whole, in order that we may concentrate all our spiritual powers upon the unseen and eternal things.
Christian fasting, at its root, is the hunger of a homesickness for God.
Author: John Piper Source: A Hunger for God, 1997, p. 14.
That's what I think fasting is at heart. It's an intensification of prayer. It's a physical explanation point at the end of the sentence, "We hunger for you to come in power." It's a cry with your body, "I really mean it, Lord! This much, I hunger for you."
Author: John Piper Source: When the Bridegroom Is Taken Away, They Will Fast - With New Wineskins, January 8, 1995
The absence of fasting is the measure of our contentment with the absence of Christ.
Author: John Piper Source: A Hunger for God, 1997, p. 93.
What are we salves to? What are our bottom-line passions? Fasting is God's testing ground - and healing ground. Will we murmur as the Israelites murmured when they had no bread? Will be leave the path of obedience and turn stones into bread? Or will we "live by every word that proceeds out of the mouth of God?" Fasting is a way of revealing to ourselves and confessing to God what is in our hearts.
Author: John Piper Source: Man Shall Not Live by Bread Alone, Sermon, January 15, 1995
Fastings and vigils without a special object in view are time run to waste.