It is necessary for a Christian to fast, in order to clear his mind, to rouse and develop his feelings, and to stimulate his will to useful activity. These three human capabilities we darken and stifle above all by "surfeiting, and drunkenness, and cares of this life."
Through these we fall away from God, the Source of life, and fall into corruption and vanity, perverting and defiling the image of God within us.
Surfeiting and sensuality nail us to the earth, and cut off, so to say, the wings of the soul.
But look how high was the flight of the souls of the ascetics and abstinent! They soared in the heavens like eagles; they, the earth-born, lived by their intellect and heart in heaven, and heard there unspeakable words, and learned there Divine wisdom.
And how a man lowers himself by gluttony and drunkenness!
He perverts his nature, created after the image of God, and becomes like unto the beast, and even worse. O, woe unto us for our attachments, for our iniquitous habits! They hinder us from loving God and our neighbors, and from fulfilling God's commandments; they implant in us criminal carnal self-love, the end of which is everlasting destruction.
Thus the drunkard does not begrudge money for the sake of gratifying his flesh and stupefying himself, while he begrudges giving a few pence to the poor.
The smoker flings away tens and hundreds of rubles, and begrudges pence to the poor, which might save his soul.
Those who love to dress luxuriously, or are lovers of elegant furniture or china, spend enormous sums upon dress, furniture, and china, and pass by beggars coldly and contemptuously.
Those who like to fare sumptuously do not begrudge spending tens and hundreds of rubles for dinners, while they begrudge a few coppers to the poor.
It is necessary for a Christian to fast, because, with the incarnation of the Son of God, human nature became spiritualized and made godly, and now we hasten towards the kingdom of God, which is “not meat and drink, but righteousness, and peace, and joy in the Holy Ghost."
"Meats for the belly, and the belly for meats: but God shall destroy both it and them."
To eat and drink—that is, to care for sensual pleasures—is only natural to heathenism, which, not knowing spiritual, heavenly delights, sets the whole life in the pleasures of the belly, in much eating and drinking.
This is why in the Gospel the Lord so often reproves this destructive passion.
Why, therefore, shall we darken and stifle our souls and kill their last spiritual powers?
St John of Kronstadt