From “The Institutes of the Christian Religion”
16. Fasting and Prayer
Hence fasting, as it is a sign of humiliation, has a more frequent use in public than among private individuals, although, as we have said, it is common to both. In regard, then, to the discipline of which we now treat, whenever supplication is to be made to God on any important occasion, it is befitting to appoint a period for fasting and prayer. Thus when the Christians of Antioch laid hands on Barnabas and Paul, that they might the better recommend their ministry, which was of so great importance, they joined fasting and prayer, (Acts 13: 3.) Thus these two apostles afterwards, when they appointed ministers to churches, were wont to use prayer and fasting, (Acts 14: 23.) In general, the only object which they had in fasting was to render themselves more alert and disencumbered for prayer. We certainly experience that after a full meal the mind does not so rise toward God as to be borne along by an earnest and fervent longing for prayer, and perseverance in prayer. In this sense is to be understood the saying of Luke concerning Anna, that she "served God with fastings and prayers, night and day," (Luke 2: 37.) For he does not place the worship of God in fasting, but intimates that in this way the holy woman trained herself to assiduity in prayer. Such was the fast of Nehemiah, when with more intense zeal he prayed to God for the deliverance of his people, (Neh. 1: 4.) For this reason Paul says, that married believers do well to abstain for a season, (1 Cor. 7: 5,) that they may have greater freedom for prayer and fasting, when by joining prayer to fasting, by way of help, he reminds us it is of no importance in itself, save in so far as it refers to this end. Again, when in the same place he enjoins spouses to render due benevolence to each other, it is clear that he is not referring to daily prayers but prayers which require more than ordinary attention.