CHAP. I. — It is not to be doubted, dearly beloved, that every Christian observance is of divine instruction, and that whatever has been received by the Church into the custom of devotion proceeds from apostolic tradition and from the teaching of the Holy Spirit; who now also presides over the hearts of the faithful by His own institutions, that all may both obediently keep them and wisely understand them.
For when on the day of Pentecost, which we celebrate as the fiftieth day from the Lord's Pasch, the Holy Spirit promised by the Lord filled the minds of those awaiting Him with a greater abundance and a clearer presence of His majesty than ever before, it is most manifestly evident that among the other gifts of God the grace of fasting also—which follows undivided upon today's festival—was then bestowed: so that, as concupiscence was the beginning of sins, so continence might be the origin of virtues. In exercising this gift of God, however, we ought not on that account to be the more sluggish because both Jews and heretics often restrain themselves from the freedom of eating, and among the pagans themselves there are certain empty fasts. For reason under truth does one thing, deception under falsehood another.
Among us faith sanctifies even the one who eats; among them unbelief defiles even the one who fasts. Hence, because outside the Catholic Church nothing is whole, nothing is chaste—as the Apostle says: 'Whatsoever is not of faith is sin' (Rom. 14:23)—we are compared by no likeness to those divided from the unity of Christ's body, nor mingled with them in any communion: which indeed is for us the most salutary and greatest fast. For nothing pertains more to the virtue of continence than to abstain from errors, since one truly walks well only when one advances along the way of truth.
For because those who, shunning the narrow and steep ways and pursuing the downward and broad ones, quickly come to destruction, a slower pace along the straight path is better than a hasty speed along a byway. Let the Catholic Christian therefore acknowledge the fruits of his fasting, which even amid the greatest liberalities will be most barren unless it proceed from the watering of the Holy Spirit. For since the Apostle says that no virtues profit him without charity (1 Cor. 13:1-3), and the same says that the charity of God is poured forth in our hearts by the Holy Spirit who is given to us (Rom. 5:5), we must beware lest we lose by pride the good things which we cannot do without goodness.
For everyone strips himself of all merit of praise who glories in himself rather than in the Lord over the pursuits of his own diligence (cf. 1 Cor. 1:31; 2 Cor. 10:17; Jer. 9:23-24): since blessed David teaches that God is to be praised in the works of the saints, saying: 'God is wonderful in his saints; the God of Israel, he will give power and strength to his people' (Ps. 67:36); and again: 'O Lord, in the light of thy countenance they shall walk, and in thy name they shall rejoice all the day, and in thy justice they shall be exalted, for thou art the glory of their strength' (Ps. 88:16-18). And therefore, dearly beloved, according to the...
...the instruction of the Holy Spirit, through whom all the gifts of the virtues have been bestowed upon the Church of God, let us undertake the solemn fast with eager faith, and in the commandments which we shall be able to fulfill let us restrain ourselves from the inflation of boasting, referring all things to the glory of God, who is both the inspirer of good wills and the author of good actions, as the Lord says: 'So let your light shine before men, that they may see your good works, and glorify your Father who is in heaven' (Matt. 5:16): who lives and reigns for ever and ever. Amen.