The first real clue to what fasting should mean today comes from a study of the Bible. For the clue is seen in the fact that in the Old Testament period fasting had an entirely different purpose than it does in the New. What is more, the pivotal text upon which this change takes place is the text we are studying in Matthew.
What was the purpose of fasting in Old Testament times? The answer is that fasting was always connected with mourning for sin and repentance of it. Thus, the entire Hebrew nation was to fast on the Day of Atonement, for this was the day in which they were to mourn for their sin and look for the reconciliation that God had provided through the sacrifices. This was the only occasion in the Old Testament on which Israel was specifically commanded to fast. From this, the practice spread to occasions of national disaster or mourning. In the book of Joel it was part of God’s call to the people to repentance (Joel 2:12). Nineveh repented with fasting after hearing the preaching of Jonah (Jonah 3:5). Israel fasted following the civil war with Benjamin (Judg. 20:26), the death of Saul and Jonathan (1 Sam. 31:13), and as part of a national revival under the prophet Samuel (1 Sam. 7:6). The Hebrew word for fasting suggests the repentant humbling of the soul before God.
If anyone doubts that this is the central characteristic of fasts in the Old Testament era, the definitive proof is found in two other texts where it is sanctioned. The first is Matthew 9:14-15, where Jesus is speaking. In this case, the disciples of John the Baptist, who practiced fasting, came to Jesus to ask why His disciples did not fast. Jesus answered, “Can the sons of the bride chamber mourn, as long as the bridegroom is with them?” In other words, the disciples did not fast because fasting implied sorrow, and the years of Jesus’ earthly ministry were to be joyous.
The other text is a great one, for it is probably the most extensive discussion of fasting in the Old Testament. It occurs in Isaiah 58:1-7. Here the people of Israel reminded God that they had fasted, but complained that fasting had not produced the results they were seeking. God answers by saying, “Behold, ye fast for strife and debate, and to smite with the fist of wickedness… 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 thy bread to the hungry and that thou bring the poor that are cast out to thy house? When thou seest the naked, that thou cover him; and that thou hide not thyself from thine own flesh?” In other words, God says that ceremonial fasting means nothing. The only fasting that is of any value is that which involves repentance of sin and that results in a transformed and charitable life. This is what fasting implied before Christ’s coming.