Fasting doesn't get much attention in the church these days, so people are unsure how we should view this. Should we fast today? What's the purpose of fasting? How long should we fast for?
Let's take a brief look at those questions, but first let me say that you shouldn't fast if you are pregnant or have some health condition that would be adversely affected by fasting. Drink plenty of water when you fast, and add some salt for electrolytes.
So should we fast?
In Matthew 6, Jesus is mentions three things and gives guidance on how they should be done. He tell us how to do our alms (acts of righteousness), how to pray, and then says starting in verse 16, "Moreover when ye fast [emphasis added], be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward. But thou, when thou fastest, anoint thine head, and wash thy face; That thou appear not unto men to fast, but unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly."
We do acts of righteousness (good works), and we pray, but fasting gets neglected! Yet, Jesus gave us guidance for when we do it. So that answers the question on if we should fast today. Another passage to look at is Matthew 9:14-15: "Then came to him the disciples of John, saying, Why do we and the Pharisees fast oft, but thy disciples fast not? And Jesus said unto them, Can the children of the bridechamber mourn, as long as the bridegroom is with them? but the days will come, when the bridegroom shall be taken from them, and then shall they fast."
So for you strict dispensationalists, Jesus is telling us we should fast after His ascension (the current church age). We also see fasting mentioned in the Book of Acts and Paul's letters to the church in Corinth.
So what is the purpose of fasting?
There are many examples given in the Old Testament, such as in Nehemiah, the Psalms, Daniel, and elsewhere. Here's a few examples that show us cause to fast:
Joel 2:12 - "Therefore also now, saith the Lord, turn ye even to me with all your heart, and with fasting, and with weeping, and with mourning."
Psalm 35:13 - "But as for me, when they were sick, my clothing was sackcloth: I humbled my soul with fasting; and my prayer returned into mine own bosom."
Isaiah 58:5 - "Is it such a fast that I have chosen? a day for a man to afflict his soul? is it to bow down his head as a bulrush, and to spread sackcloth and ashes under him? wilt thou call this a fast, and an acceptable day to the Lord?"
Daniel 9:3 - "And I set my face unto the Lord God, to seek by prayer and supplications, with fasting, and sackcloth, and ashes."
So fasting can be done for mourning or humbling yourself before God to desperately seek Him. Fasting should be done to dedicate time to God, to give yourself to prayer, and humble yourself in total reliance on the Almighty. When you fast, and feel your stomach growling, set your mind to the Lord to worship him and thank Him for His many blessings.
And how long should we fast for?
Well, there isn't any rule on this but let's look at a couple examples.
We all know Jesus fasted for 40 days in the wilderness, but that is not recommended. Don't try it. Seriously.
Daniel fasted for 21 days (and here's an interesting study on a modified "Daniel Fast" if you're interested: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2941756/), "In those days I Daniel was mourning three full weeks. I ate no pleasant bread, neither came flesh nor wine in my mouth, neither did I anoint myself at all, till three whole weeks were fulfilled" (Daniel 10:2-3).
And the Pharisees fasted twice a week (presumably went without food for a day at a time). Luke 18:12, "I fast twice in the week, I give tithes of all that I possess."
My suggestion is to do it for one day at first, and make sure you have time that day to spend in prayer. Understand how your body feels and then you can increase from there.
Here are some helpful quotes on fasting:
So again, don"t draw attention to yourself or don"t be doing it in such a way that you are receiving from man that awe and wonder because you are so spiritual because you fast. It used to be in the earlier days of my ministry that I fasted quite often. Obviously I don"t fast much anymore.
"But in those early days of the ministry, many times I"d be fasting and I"d go on my pastoral calls and some sweet lady would offer me a piece of homemade cake, you know. And I was fasting you know, for the last couple of days and what are ya gonna do? Well, thank you but I"m fasting. No, I"d just break the fast and eat the cake. Because I figure, hey, if you go ahead and just say, oh no I can"t eat that I"m fasting, then you have your reward anyhow. And I wasn"t really looking for the reward of man. I was wanting, you know, to receive the reward from God. And so offered something to eat, if I could get out of it I would, but you know, when they bake a cake you can"t really get out of it. They want their reward, oh this cake is delicious, you know." - Chuck Smith
"...Fast means, a total abstinence from food for a certain time. Abstaining from flesh, and living on fish, vegetables, etc., is no fast, or may be rather considered a burlesque on fasting. Many pretend to take the true definition of a fast from Isaiah 58:3, and say that it means a fast from sin. This is a mistake; there is no such term in the Bible as fasting from sin; the very idea is ridiculous and absurd, as if sin were a part of our daily food. In the fast mentioned by the prophet, the people were to divide their bread with the hungry, Isaiah 58:7; but could they eat their bread, and give it too? No man should save by a fast: he should give all the food he might have eaten to the poor. He who saves a day's expense by a fast, commits an abomination before the Lord." - Adam Clarke