Saint Peter Chrysologus (c.406-450), Bishop of Ravenna, Doctor of the Church Sermon on prayer, fasting and almsgiving; PL 52, 320
There are three actions, my brethren, on which faith stands firm, in which piety consists and by which virtue is upheld: prayer, fasting, mercy. Prayer knocks at the door; fasting obtains; mercy receives. Prayer, mercy, fasting: these three make up one thing and, all together, give life to each other. Thus, fasting is the soul of prayer and mercy is the life of fasting. Let no one set them apart; the three together cannot be separated. Anyone who puts only one or two of them into practice has done nothing at all. Therefore, the one who prays must fast, and the one who fasts must have mercy. Let him hear the man who asks and who, in asking, wishes to be heard. Whoever does not refuse to listen to others when they plead to him will be listened to by God. Whoever practises fasting must understand what fasting is, that is to say, he must sympathise with the hungry man if he wants God to sympathise with his own hunger. Whoever hopes to obtain mercy must show mercy; whoever wants to benefit from kindness must practise it; whoever would like someone to give him something must be someone who gives… So be yourself the measure of the mercy to be shown to you: if you would like others to show you mercy in such-and-such a way, according to such a measure and with just such a readiness, then show mercy yourself to others with the same sort of readiness, according to the same measure, and in the same way. And so prayer, mercy, fasting must make up one, single sponsor to recommend us to God, one defense, one prayer in our favour under this threefold form.