1.1 The Essence of Christian Fasting
"Fasting has an important place in all the great religions. The Old Testament lists fasting among the corner-stones of the spirituality of Israel: "Prayer is good when accompanied by fasting, almsgiving and justice" (Tob 12:8 NAB).* Fasting implies an attitude of faith, humility and complete dependence upon God. Fasting is used to prepare to meet God (cf. Ex 34:28; 1 Kgs 19:8; Dan 9:3); to prepare for a difficult task (cf. Jgs 20:26; Es 4:16) or to seek pardon for an offence (cf. 1 Kgs 21:27); to express grief in the wake of domestic or national misfortune (cf. 1 Sam 7:6; 2 Sam 1:12; Bar 1:5 NAB). Fasting, inseparable from prayer and justice, is directed above all to conversion of heart, without which, as the Prophets declared (cf. Is 58:2-11; Jer 14:12; Zech 7:5-14), it is meaningless.
Before beginning his public mission, Jesus, driven by the Holy Spirit, fasted for forty days as an expression of his trusting abandonment to the Father's saving plan (cf. Mt 4:1-4). He gave precise instructions to his disciples that their fasting should never be tainted by ostentation and hypocrisy (cf. Mt 6:16-18).
Following the biblical tradition, the Fathers held fasting in high esteem. In their view, the practice of fasting made the faithful ready for nourishment of another kind: the food of the Word of God (cf. Mt 4:4) and of fulfillment of the Father's will (cf. Jn 4:34). Fasting is closely connected to prayer, it strengthens virtue, inspires mercy, implores divine assistance and leads to conversion of heart. It is in this double sense, imploring the grace of the Almighty and profound inner conversion, that we are called to accept Pope John Paul II's invitation to fast [for peace.]. For without the Lord's help it will not be possible to find a solution to the tragic situation now facing the world, and it is hard to see how terrorism will be tackled at its roots without a conversion of hearts.
The practice of fasting looks to the past, present and future: to the past, as a recognition of offenses committed against God and others; to the present, in order that we may learn to open our eyes to others and to the world around us; to the future, in order that we may open our hearts to the realities of God and, by the gift of divine mercy, renew the bond of communion with all people and with the whole of creation, accepting the responsibility which each of us has in history." OFFICE OF PAPAL LITURGICAL CELEBRATIONS - Christian Fasting The Vatican.
*For many centuries, the Roman Liturgy on Ash Wednesday at the start of Lent took as the Gospel reading Matt 6:1-6, 16-18, which presents Jesus' teaching on almsgiving (mercy), prayer and fasting, which are in fact inseparable. "These three things, prayer, fasting and mercy, are a single thing, each drawing life from the others. Fasting is the soul of prayer and mercy the life of fasting. Let no one divide them, because alone they do not survive" (Saint Peter Chrysologus, Discourse 43: PL 52, 320).
See Days of Penance in the Code of Canon Law, and Forms of Penance in the Catechism.
Works Of Satisfaction Are Of Three Kinds
"Pastors should teach that all kinds of satisfaction [for sin] 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. " Council
of Trent on the Sacrament of Penance in the Roman Catechism.
See also, Fasting during prayer, and St. Thomas Aquinas on fasting in the Summa Theologica.