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The Primacy of Fasting
The Church teaches us, in no uncertain terms, that fasting is the highest external penance that anyone can perform, and thus should be preferred above all others. In fact, the Church even goes so far as to indicate that those who do not fast--whose bellies are continually satiated--are incapable of prayer! Indeed, there is a good reason why we as Christians often hear the words "prayer and fasting" together, as if this is somehow the answer to every problem (those who do not fast cannot understand the efficacy of such a prescription). We might even go so far as to say that fasting and prayer are so deeply connected, that they are virtually inseparable from one another.
Catechism of the Council of Trent: "All kinds of satisfaction are reducible to three heads: prayer, fasting and almsdeeds, which correspond to three kinds of goods which we have received from God; those of the soul, those of the body and what are called external goods. Nothing can be more effectual in uprooting all sin from the soul than these three kinds of satisfaction. For since whatever is in the world is the concupiscence of the flesh, the concupiscence of the eyes, and the pride of life, everyone can see that to these three causes of disease are opposed also three remedies. To the first is opposed fasting; to the second, almsdeeds; to the third, prayer. Moreover, if we consider those whom our sins injure, we shall easily perceive why all kinds of satisfaction are reduced especially to these three. For those (we offend by our sins) are: God, our neighbour and ourselves. God we appease by prayer, our neighbour we satisfy by alms, and ourselves we chastise by fasting. [...] This triple remedy was, therefore, appointed by God to aid man in the attainment of salvation. For by sin we offend God, wrong our neighbour, or injure ourselves. The wrath of God we appease by pious prayer; our offences against man we redeem by almsdeeds; the stains of our own lives we wash away by fasting. [...] Fasting is most intimately connected with prayer. For the mind of one who is filled with food and drink is so borne down as not to be able to raise itself to the contemplation of God, or even to understand what prayer means."
Baltimore Catechism: "The chief means by which we satisfy God for the temporal punishment due to sin are: Prayer, Fasting, Almsgiving, all spiritual and corporal works of mercy, and the patient suffering of the ills of life."
There is a good reason why the Church prescribes fasts on the vigils of great feasts, says Venerable Bellarmine, "so that Christians might be more fit for celebrating the divine solemnities." For this reason, fasting is also mandated before the reception of Holy Communion. Not only has fasting enjoyed a long-standing place of honor throughout Church history, but it is also mentioned more times in Sacred Scripture than any other penance. Additionally, fasting held such a high place in the Church, that laymen who failed to fast during Lent were excommunicated; and clerics who failed in this way incurred the degradation of their orders. It should be no surprise that many of the greatest mystics in the Church had stomach illnesses that prevented them from keeping down food (Padre Pio, Saint Jean Vianney, Blessed Anne Catherine Emmerich, Saint Faustina, etc.). In this way, Our Lord aided their austerity in fasting so that they could rise more quickly in sanctity. Fasting is also the only traditional penance that is angelic, since it closely imitates the life of the angels and saints in heaven, who neither eat nor require food. Regarding the angelic life, the Catholic Encyclopedia elaborates; "It is primarily as a means to this end that fasting takes so important a place in the monastic life. [...] Among the early...monks, fasting was carried to such lengths that some modern writers have been led to regard it almost as an end in itself". It is thus that fasting should be the preferred penance of choice in religious life, though other penances may also be encouraged.
Saint Gregory: "It is impossible to engage in spiritual conflict, without the previous subjugation of the appetite."
Saint John Chrysostom: "Fasting is the support of our soul: it gives us wings to ascend on high, and to enjoy the highest contemplation! [...] God, like an indulgent father, offers us a cure by fasting."
Saint Alphonsus De Ligouri: "He that gratifies the taste will readily indulge the other senses; for, having lost the spirit of recollection, he will easily commit faults, by indecent words and by unbecoming gestures. But the greatest evil of intemperance, is that it exposes chastity to great danger. 'Repletion of the stomach,' says St. Jerome, 'is the hotbed of lust.'
Ven. Mary of Agreda: "Temperance includes the two virtues of abstinence and sobriety...Abstinence also includes fasting. These virtues take the first place in treating of temperance; for nourishment, being necessary for the preservation of life, is among the principal objects coveted by the appetites."
Saint Basil: "Penance without fasting is useless and vain; by fasting [we] satisfy God."
Saint Catharine of Sienna: "without mortifying the taste, it is impossible to preserve innocence, since it was by the indulgence of his appetite that Adam fell."
Saint Augustine: "But now the necessity of habit is sweet to me, and against this sweetness must I fight, lest I be enthralled by it. Thus I carry on a daily war by fasting, constantly bringing my body into subjection...And while health is the reason for our eating and drinking, yet a perilous delight joins itself to them as a handmaid; and indeed, she tries to take precedence in order that I may want to do for her sake what I say I want to do for health’s sake....These temptations I daily endeavor to resist and I summon thy right hand to my help and cast my perplexities onto thee."
Saint Francis De Sales: "besides the ordinary effect of fasting in raising the mind, subduing the flesh, confirming goodness, and obtaining a heavenly reward, it is also a great matter to be able to control greediness, and to keep the sensual appetites and the whole body subject to the law of the Spirit; and although we may be able to do but little, the enemy nevertheless stands more in awe of those whom he knows can fast."
Saint Peter Chrysologus: “Fasting is the soul of prayer, mercy is the lifeblood of fasting. So if you pray, fast; if you fast, show mercy; if you want your petition to be heard, hear the petition of others. If you do not close your ear to others, you open God’s ear to yourself”
Didache: "pray for your enemies, and fast for those who persecute you."