What does the Bible say about Christian fasting?
The Bible does not actually command fasting. Fasting is not something God requires or demands of Christians. However, the Bible does say that fasting is a profitable and beneficial spiritual discipline. In the book of Acts, believers in the new church fasted together before making important decisions (Acts 13:2, 14:23), and fasting is often combined with prayer, especially prayers of particular import (Luke 2:37, 5:33). When most people think of fasting, they think of denying oneself food for a period of time. But the main purpose of fasting is not to increase one's hunger, in a self-punishing way, but to increase one's focus on God. Fasting is meant to express to God, and to ourselves, that our relationship with Him is our main need and nourishment. Fasting gives us a picture of our ultimate dependence upon God.
In Scripture, fasting is almost always done by abstaining from food. However, it is possible to fast in other ways. Anytime we give something up, temporarily, to focus exclusively on God and knowing Him and knowing His will, can be considered fasting (1 Corinthians 7:1-5). When fasting, especially from food, it is wise to limit the time spent in the fast. Long periods of time spent without eating can be counter-productive and even dangerous to one's health. Also, fasting is not "dieting". The correct motive for fasting is to grow nearer to God, rather than to lose weight. In addition, it is important to consider that some people may not be able to fast from food – those with diabetes or other digestive disorders – and God does not want us to harm or maim ourselves in an attempt to prove our allegiance to Him. Other forms of fasting are just as acceptable to God.
The idea is that by taking our eyes off of the world, and our fleshly, bodily desires, we can increase our attention to the Lord. Fasting should not be thought of as a way to manipulate God, or to barter with Him. The exercise of fasting is meant to change our hearts and perspective, not change God's mind or His will. Fasting should be seen as a spiritual exercise in giving something up to express dependence on our Creator – not an attempt to prove our allegiance to Him, or to show off our spirituality. Matthew 6:16-18 tells us, "And when you fast, do not look gloomy like the hypocrites, for they disfigure their faces that their fasting may be seen by others. Truly, I say to you, they have received their reward. But when you fast, anoint your head and wash your face, that your fasting may not be seen by others but by your Father who is in secret. And your Father who sees in secret will reward you."
We know from this verse that God's purpose for fasting is not for us to lord it over others or prove how much we are willing to suffer for God, but simply to grow nearer to Him and experience the resultant peace and joy that is our reward.
Source: Compelling Truth