In order to find biblical information about a topic, it is a good idea to use study helps such as Strong’s Exhaustive Concordance of the Bible. A concordance lists every single time a specific word is used in various translations. Reviewing the words “fast” and “fasting” in Strong’s (King James Version), we find there are no explicit directions for fasting found in God’s Law or in the first five books of the Bible. 

People fasted throughout the Bible in order to take their minds off of their bodies and to refocus their minds on spiritual matters. In the Old Testament, there are accounts of people fasting while waiting for an answer to prayer. See Ezra 8:21, 9:5, Nehemiah 1:4, Esther 4:3,16. Many times people would fast because they had sinned. They used fasting to purify their bodies and to humble themselves before God. See Nehemiah 9:1, Daniel 9:3.

However, Isaiah 58 records God’s quarrel with Israel and the manner of their fasting. “…declare to My people their transgression… (Israel said) Why have we fasted, and you see it not? Why have we humbled ourselves, and you take no knowledge of it? (God answers) Behold, in the day of your fast you seek your own pleasure and oppress all your workers. Behold, you fast only to quarrel and to fight and to hit with a wicked fist. Fasting like yours this day will not make your voice to be heard on high. Is such the fast that I choose, a day for a person to humble himself? Is it to bow down his head like a reed, and to spread sackcloth and ashes under him? Will you call this a fast, and a day acceptable to the LORD? Is not THIS the fast that I choose: to loose the bonds of wickedness, to undo the straps of the yoke, to let the oppressed go free, and to break every yoke? Is it not to share your bread with the hungry and bring the homeless poor into your house; when you see the naked, to cover him, and not to hide yourself from your own flesh?” Isaiah 58:1-7

From God’s perspective, God wants us to sacrifice our time and efforts to do His will and not our own. Denial of the flesh through fasting and to sit in sackcloth and ashes (which was the custom) without a true heart transformation doesn’t impress God.

However, Jesus fasted for 40 days in the wilderness. In this account, it does not say why Jesus fasted. He was not fasting due to sin or to wait for an answer to prayer, but likely he abstained from food to focus on his intense communion with his Father. Jesus was preparing for his ministry, and it’s reasonable to believe he was focusing on the details of how he was going to sacrifice his life as a ransom for all (1 Timothy 2:6).

Additional Resources:
Christian Questions Podcast
Episode #1243: “Am I Putting New Wine in an Old Wineskin?”
Three parables to help us stretch with the Christian development required of us
Preview Video
CQ Rewind Show Notes

Episode #826: “Why Focus on Fasting and Prayer?”
The role of fasting (particularly) and praying in our lives
CQ Rewind Show Notes