Origin of Biblical Fasting:

The Fasts of Moses

The first inference to fasting is found in Exodus, during Moses’ second iteration of the Ten Commandments. I say inference because “fast/fasting” is not used.

“So he was there with the LORD forty days and forty nights. He neither ate bread nor drank water. And he wrote on the tablets the words of the covenant, the Ten Commandments” (Exodus 34:28).

It’s commonplace in the Bible for bread to refer to all foods, provisions. Remember Acts 2:42: “And they devoted themselves to the teaching and the fellowship, to the breaking of the bread and the prayers” (emphasis mine). Although the breaking of bread can also refer to the bread of communion, in this instance it refers to believers sharing meals with one another. Much like today when we say, “Let’s break bread together.” What we mean is “let’s have a meal together.” 

So, Moses ate nothing for forty days. Nor did he drink anything for forty days. That he drank no water implies that he drank no liquids of any kind. 

In Deuteronomy 9, Moses reminds the Israelites of God’s anger toward them for their disobedience and recounts the Exodus 34:28 narrative, but first informs them of his first 40-day fast during his first communion with God on Mount Sinai, also known as Mount Hereb:

“Remember how you made the LORD your God angry in the desert. Never forget that! From the day you left the land of Egypt to the day you came to this place, you have refused to obey the LORD. You made the LORD angry at Mount Horeb. The LORD was angry enough to destroy you! I went up to the mountain to get the stone tablets. The agreement the LORD made with you was written on those stones. I stayed on the mountain 40 days and 40 nights. I did not eat any food or drink any water” (Deuteronomy 9:7-9).

Two verses later, he says this:

“So at the end of 40 days and 40 nights, the LORD gave me the two stone tablets ~ the stones of agreement. Then the LORD said to me, ‘Get up and quickly go down from here. The people you brought out of Egypt have ruined themselves. They stopped obeying my commands so quickly. They melted gold and made an idol for themselves” (Deuteronomy 9:11-12).

In verse 14 we find out that God is so angry that he says:

“Let me destroy these people completely, so no one will even remember their names. Then I will make another nation from you that is stronger and greater than these people” (Deuteronomy 9:14).

Moses proceeds to go down the mountain, sees the golden calf, sees the rampant disobedience, and in anger throws down and breaks the tablets in front of them. Then he informs the people of his second 40-day fast, but in this account (which is told in first person, unlike Exodus 34:28) he includes the reason for his fast:

“Then I bowed down before the LORD with my face to the ground for 40 days and 40 nights, as I did before. I did not eat any food or drink any water. I did this because you had sinned so badly. I was afraid of the LORD’s terrible anger. He was angry enough to destroy you, but the LORD listened to me again. The LORD was very angry with Aaron ~ enough to destroy him! So I also prayed for Aaron at that time” (Deuteronomy 9:18-20).

In verse 25 Moses tells of his third 40-day fast. The reason is the same as the second: he feared the LORD would destroy them. But during this fast, Moses not only pleaded with the LORD not to destroy them, but also reminded him of his promise:

“Remember your promise to your servants Abraham, Isaac, and Jacob. Forget how stubborn these people are. Don’t look at their evil ways or their sins. If you punish your people, the Egyptians might say, ‘The LORD was not able to take his people into the land he promised them. And he hated them. So he took them into the desert to kill them.’ But they are your people, they belong to you. You brought them out of Egypt with your great power and strength” (Deuteronomy 9: 27-29).