From The Christian’s Daily Walk
A religious fast is sanctifying a day to the Lord by a willing abstinence from meat [food] and drink and from delights and worldly labours, that the whole man may be more thoroughly humbled before God and more fervent in prayer. This fast has two parts: the one, outward, chastening the body; the other, inward, the afflicting of the soul; under which are contained all those religious acts which concern the setting of the heart right towards God, and the seeking help of God for those things for which the fast is intended.
Abstaining from Food & Drink, without an Inward Work, is Not a Religious Fast
If you fast strictly for bodily abstinence, then it is an indifferent thing and is no part of God’s worship. But if it is joined with the inward part, and is referred to a religious end, being a profession of an extraordinary humiliation, then it is a great assistance to a man’s spiritual and reasonable service of God. It gives a stronger and speedier wing to prayer, which must always go with it, Ezra 8:23. Ps 35:13; so it is more than an ordinary worship. Fasting has the name from the outward part, Mark 9:29, 1st Cor 7:5, Acts 13:3; it being most sensible [noticeable]; but its excellency and efficacy is from the inward, being that [purpose] for which the outward is observed.
Public and Private Fasts
A fast is called public when a whole state or when any one public congregation doth fast. [It is called] private when one alone, one family, or some few together, do fast.
Examples from Scripture
Public and private fasts have their warrant from the New Testament, as well as from the Old, which shows that religious fasts were not peculiar to the Jews, but are a Christian duty, belonging to all fitly qualified for them.
In the sacred Scriptures we have manifold examples of private fasts and examples and commandment for public ones. Our Lord and Saviour said that his disciples, after his departure from them, should fast, Matt 9:15, Matt 6: 16-17, and gives directions unto all concerning private fasts. The apostle speaks of husbands and wives abstaining from conjugal embraces, that they might give themselves to fasting and prayer, 1 Cor 7:5. And we have repeated examples of the apostles and primitive Christians for religious fasts, Acts 13: 2-3, Acts 14: 23. All which prove fasting to be a Christian duty.
The case of a person’s self, family, the church, or commonwealth, may be such that ordinary humiliation and prayer will not suffice. For as there were some devils that could not be cast out but by fasting and prayer, Mark 9:29, so it may be that such hardness of heart may be grown upon a person, or some sinful lusts may have gotten so much strength, that they will not be subdued; some evils, private and public (1 Sam. 7:5,7; Judges 20:18, 23, 26) which cannot be prevented or removed; some special graces and blessings which shall not be obtained or continued, but with the most importunate seeking of God by fasting and prayer.