St. Ambrose of Milan, Epistle to the Church of Vercellae 63, 17 (396
AD)
Who then are these new teachers who reject the merit of fasting? Is it not
the voice of heathen who say, Let us eat and drink? whom the Apostle
well ridicules, when he says: If after the manner of men I have fought with
beasts at Ephesus, what advantage is it to me if the dead rise not?
And,
consequently, if all hope of the resurrection is lost, let us eat and drink, let
us not lose the enjoyment of things present, who have none of things to
come. It is then for them to indulge in meats and drinks who hope for
nothing after death.
St. Leo I, Sermon 12, 4 (inter 440-461 AD)
This threefold round of duty, dearly beloved, brings all other virtues into
action: it attains to Gods image and likeness and unites us inseparably with
the Holy Spirit. Because in prayer faith remains steadfast, in fastings life
remains innocent, in almsgiving the mind remains kind. On Wednesday and
Friday therefore let us fast: and on Saturday let us keep vigil with the most
blessed Apostle Peter, who will deign to aid our supplications and fast and
alms with his own prayers through our Lord Jesus Christ, who with the
Father and the Holy Ghost lives and reigns for ever and ever. Amen.
Catechism of the Council of Trent (1566)
Pt. IV, Ch. VI: To prayer let us unite fasting and almsdeeds. 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.
Catechism of the Catholic Church (1992)
No. 1434:
The interior penance of the Christian can be expressed in
many and various ways. Scripture and the Fathers insist above all on three
forms, fasting, prayer, and almsgiving, which express conversion in
relation to oneself, to God, and to others. Alongside the radical purification
brought about by Baptism or martyrdom they cite as means of obtaining
forgiveness of sins: efforts at reconciliation with ones neighbor, the
intercession of the Saints, and the practice of charity which covers a
multitude of sins.