Sayings of the Holy Fathers:

Fasting is an exceptional virtue; it represses bodily impulses and gives strength to the soul to fight against the poisoning of the heart through the senses, and provides it with a remedy against any past poisoning. Fasting causes the mind to be cleansed constantly. It whithers up every evil thought and brings healthy, godly thoughts -- -holy thoughts that enlighten the mind and kindle it with more zeal and spiritual fervor.

Elder Ephraim of Philotheou Mount Athos, 
"Counsels from the Holy Mountain" 

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A life of fasting, properly understood as general self-limitation and abstinence, to the annual practice of which the Church always calls us with the Great Lent, is really that bearing of the cross and self-crucifixion which is required of us by our calling as Christians. And anyone who stubbornly resists this, wanting to live a carefree, happy, and free life, is concerned for sensual pleasures and avoids sorrow and suffering that person is not a Christian. Bearing one's cross is the natural way of every true Christian, without which there is no Christianity.  

Archbishop Averky of Syracuse (of Blessed Memory)

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A worker takes the trouble to get hold of the instruments that he requires. He does so not simply to have them and not use them. Nor is there any profit for him in merely possessing the instruments. What he wants is, with their help, to produce the crafted objective for which these are the efficient means.

In the same way, fasting, vigils, scriptural meditation, nakedness and total deprivation do not constitute perfection but are the means to perfection. They are not in themselves the end point of a discipline, but an end is attained to through them.  

St. John Cassian, Conference One