Fausset's Bible Dictionary
The word (tsum ) never occurs in the Pentateuch. The Mosaic law, though directing minutely the foods to be eaten and to be shunned, never enjoins fasting. The false asceticism so common in the East was carefully avoided. On the yearly day of atonement, the 10th day of the 7th month, Israelites were directed to "afflict the soul" (Leviticus 16:29-31; Leviticus 23:27; Numbers 30:13). This significant term implies that the essence of scriptural "fasting" lies in self humiliation and penitence, and that the precise mode of subduing the flesh to the spirit, and of expressing sorrow for sin, is left to the conscientious discretion of each person. In Acts 27:9 the yearly day of atonement is popularly designated "the fast."
But God, while not discountenancing outward acts of sorrow expressive of inward penitence, declares, "is not this the fast that I have chosen? to loose the bands of wickedness, to undo the heavy burdens, and to let the oppressed go free, and that ye break every yoke? Is it not to deal the bread to the hungry, and that thou bring the poor that are cast out to thy house? when thou seest thy naked that thou cover him, and that thou hide not thyself from thine own flesh?" (Isaiah 58:4-7.) Compare similar warnings against mistaking outward fasting as meritorious before God: Malachi 3:14; Matthew 6:16.
The only other periodical fasts in the Old Testament were those connected with the capture of Jerusalem by Nebuchadnezzar: the fast of the 4th month commemorated its capture (Jeremiah 39:2; Jeremiah 52:6-7); that of the 5th month the burning of the temple and the chief houses (Jeremiah 52:12-14); that of the 7th the murder of Gedaliah (Jeremiah 41:1-3); that of the 10th the beginning of the siege (Zechariah 7:3-5; Zechariah 8:19). Jeremiah 52:4, "did ye at all fast unto ME, even to ME?" Nay, it was to gratify yourselves in hypocritical will worship. If it had been to Me, ye would have separated yourselves not merely from food but from your sins.
Once that the principle is acted on, "he that eateth eateth to the Lord, and he that eateth not, to the Lord he eateth not" (Romans 14:6), and "meat commendeth us not to God, for neither if we eat are we the better, neither if we eat not are we the worse" (1 Corinthians 8:8), fasting and eating are put in their true place, as means not ends. There are now 28 yearly fasts in the Jewish calendar. Daniel's (Daniel 10:3) mode of fasting was, "I ate no pleasant bread," i.e. "I ate unleavened bread, even the bread of affliction" (Deuteronomy 16:3), "neither came flesh nor wine in my mouth." In Matthew 9:14 "fast" is explained by "mourn" in Matthew 9:15, so that fasting was but an outward expression of mourning (Psalms 69:10), not meritorious, nor sanctifying in itself.
A mark of the apostasy is "commanding to abstain from meats which God hath created to be received with thanksgiving" (1 Timothy 4:3). The "neglecting (not sparing) of the body," while seeming to deny self, really tends "to the satisfying of (satiating to repletion) the flesh." Ordinances of "will worship" gratify the flesh (self) while seeming to mortify it; for "self crowned with thorns in the cloister is as selfish as self crowned with ivy in the revel" (Colossians 2:18-23). Instances of special fasts of individuals and of the people in the Old Testament, either in mourning and humiliation or in prayer, occur in Judges 20:26; 1 Samuel 1:7; 1 Samuel 20:34; 1 Samuel 31:13; 2 Samuel 1:12; 2 Samuel 12:21; 2 Samuel 3:35; 1 Kings 21:9-12; Ezra 8:21-23; Ezra 10:6; Esther 4:16; Nehemiah 1:4.
National fasts are alluded to in 1 Samuel 7:6 (wherein the drawing of water and pouring it out before Jehovah expressed their confession of powerlessness and utter prostration: Psalms 22:14; Psalms 58:7; 2 Samuel 14:14); 2 Chronicles 20:3; Jeremiah 36:6-10; Nehemiah 9:1; Joel 1:14; Joel 2:15. In New Testament times the strict Jews fasted twice a week (Luke 18:12), namely, on the second and fifth days. While Christ is with His people either in body or in spirit, fasting is unseasonable, for joy alone can be where He is; but when His presence is withdrawn, sorrow comes to the believer and fasting is one mode of expressing his sorrowing after the Lord. This is Christ's teaching, Matthew 9:15. As to the texts quoted for fasting as a mean of spiritual power, the Sinaiticus and Vaticanus manuscripts omit Matthew 17:21; they omit also "and fasting," Mark 9:29. They and Alexandrinus manuscript omit "fasting and," 1 Corinthians 7:5. Evidently the growing tendency to asceticism in post apostolic times accounts for these interpolations.
The apostles "prayed with fasting" in ordaining elders (Acts 13:3; Acts 14:23). But this continuance of the existing Jewish usage never divinely ordered does not make it obligatory on us, except in so far as we severally, by experience, find it conducive to prayer. Moses', Elijah's, and Christ's (the great Antitype) 40 days' foodlessness was exceptional and miraculous. Forty is significant of punishment for sin, confession, or affliction. Christ, the true Israel, denied Himself for 40 days, as Israel indulged the flesh 40 years. They tempted God that time; He overcame the tempter all the 40 days (Genesis 7:4; Genesis 7:12; Numbers 14:33; Numbers 32:13-14; Psalms 95:10; Deuteronomy 25:3; 2 Corinthians 11:24; Ezekiel 29:11; Ezekiel 4:6; Jonah 3:4).
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Fausset, Andrew R. Entry for 'Fasting'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/f/fasting.html. 1949.