The Mosaic Law ordered the Jews to “afflict [their] souls,” that is, to fast, once a year on Atonement Day. (Leviticus 16:29-31; Psalm 35:13) This was the only fasting that Jehovah ever commanded his people to do.* Jews who lived under the Mosaic Law would have obeyed that command. But Christians are not required to observe the Mosaic Law.—Romans 10:4; Colossians 2:14. Although Jesus did fast as the Law required, he was not known for this practice. He told his disciples how they were to act if they chose to fast, but he never commanded that they fast. (Matthew 6:16-18; 9:14) Why, then, did Jesus say that his disciples would fast after his death? (Matthew 9:15) This was not a command. Jesus’ words simply suggest that at his death his disciples would feel deep sorrow and would lose the desire to eat. Two Biblical accounts of early Christians who did fast show that if with good motive a person chooses to abstain from food, this is acceptable to God. (Acts 13:2, 3; 14:23) Christians, then, are under no obligation to fast. Yet, a person who chooses to do so should be alert to certain dangers. One pitfall to avoid regarding fasting is self-righteousness. The Bible warns against adopting “mock humility.” (Colossians 2:20-23) Jesus’ illustration of the proud Pharisee who felt morally superior to others because of his regular fasting leaves no doubt that God rejects such an attitude.—Luke 18:9-14. It would also be a mistake to publicize the fact that you fast or to fast because another person tells you to do so. According to Matthew 6:16-18, Jesus counseled that fasting should be a private matter, between you and God, and that you should not announce it to others. One should never think that fasting somehow compensates for sinning. To be acceptable to God, a fast must be accompanied by obedience to his laws. (Isaiah 58:3-7) Heartfelt repentance, not the act of fasting itself, is what leads to the forgiveness p. 29of sins. (Joel 2:12, 13) The Bible emphasizes that we receive forgiveness by Jehovah’s undeserved kindness expressed through the sacrifice of Christ. It is impossible to earn forgiveness through any works, including fasting.—Romans 3:24, 27, 28; Galatians 2:16; Ephesians 2:8, 9. Isaiah 58:3 illustrates another common error. The Israelites suggested that Jehovah owed them something in return for their fasting, as if by fasting, they were doing God a favor. They asked: “For what reason did we fast and you did not see, and did we afflict our soul and you would take no note?” Many today likewise think that because of their fasting, they can expect God to perform some favor for them in return. May we never imitate such a disrespectful and unscriptural attitude! Others believe that it is possible to earn merit by submitting the body to discomfort through fasting, whipping themselves, or the like. God’s Word condemns this notion, showing that “a severe treatment of the body” is “of no value in combating” wrong desires.—Colossians 2:20-23. Fasting is not obligatory; nor is it wrong. It may be beneficial in some circumstances if the dangers mentioned above are avoided. Fasting, however, is not the focus of acceptable worship. Jehovah is “the happy God,” and he wants his servants to be happy. (1 Timothy 1:11) His own Word says: “There is nothing better for them than . . . that every man should eat and indeed drink and see good for all his hard work. It is the gift of God.”—Ecclesiastes 3:12, 13. Our worship should be characterized by joy, but the Bible never associates fasting with happiness. Moreover, if abstaining from food were to affect our health adversely or sap our energy for the joyful work that our Maker has entrusted to true Christians—that of declaring the good news of the Kingdom—then it would clearly be counterproductive. Whether we choose to fast or not, we should avoid judging others. Among true Christians, there should be no controversy over this subject, “for the kingdom of God does not mean eating and drinking, but means righteousness and peace and joy with holy spirit.”—Romans 14:17.
Hope this helps.
The Bible and www.jw.org