What is fasting in the Bible?
Short answer
Fasting in the Bible is giving up food for a set time in order to give attention to God, almost always alongside prayer. It appears in mourning, in repentance and before important decisions. Jesus fasted forty days and taught fasting without display, and Isaiah 58 ties true fasting to justice toward one’s neighbour.
- Written by
- Maria Luisa Navarro Schmitt
- Reviewed by
- Raphael Navarro Schmitt
- Published on
- Reading time
- 3 min read
Biblical fasting is neither a diet nor an isolated penance. It hardly ever appears on its own: it comes attached to prayer, to mourning, to collective repentance or to a decision the community needs to make. In Acts, the church at Antioch fasts before sending out Barnabas and Saul, and fasts again at the moment of sending them off.
There is no obligatory fasting calendar for the Christian in the Bible, and no fixed duration. What exists are examples of very different lengths, from a one-day fast to the forty days Jesus spends in the desert before beginning to preach — and that last one is narrated as preparation, not as a requirement to be imitated literally.
The instruction of Jesus on the subject is short and deals with one risk only: fasting in order to be noticed. He directs the opposite of what the practice tends to produce — tidy yourself, wash your face, carry on with the day as normal. A fast that shows on the outside has, in the text, already collected its payment in advance.
There is also a question his own contemporaries asked: why the disciples of Jesus did not fast the way John the Baptist’s and the Pharisees’ did. His answer shifts the axis of the discussion from rule to time — while the groom is present, there is no fasting; days will come when he is taken away, and then there will be. Fasting, on that logic, answers to a situation rather than to a date on the calendar.
The severest text on the subject is from Isaiah, and it does not question the practice but the result. A people that fasts and goes on oppressing is described as keeping the wrong fast; the right one is defined by releasing constraints, breaking bread with the hungry and taking in the homeless. It is the criterion the Bible offers for assessing your own fast, and it points outward from whoever is fasting.
What the passages say
16And when you fast, do not choose to become gloomy, like the hypocrites. For they alter their faces, so that their fasting may be apparent to men. Amen I say to you, that they have received their reward.
17But as for you, when you fast, anoint your head and wash your face,
18so that your fasting will not be apparent to men, but to your Father, who is in secret. And your Father, who sees in secret, will repay you.
The only direct instruction of Jesus about how to fast, and it is entirely about discretion. It sits in the same block of the Sermon on the Mount that deals with almsgiving and prayer, with the same structure: a legitimate practice ruined when it turns into a display.
1Then Jesus was led by the Spirit into the desert, in order to be tempted by the devil.
2And when he had fasted for forty days and forty nights, afterwards he was hungry.
The longest fast in the New Testament, placed before the start of the public ministry. The text presents it as preparation for a specific moment — which is different from presenting it as a measure of spirituality to be repeated.
18And the disciples of John, and the Pharisees, were fasting. And they arrived and said to him, “Why do the disciples of John and of the Pharisees fast, but your disciples do not fast?”
19And Jesus said to them: “How can the sons of the wedding fast while the groom is still with them? During whatever time they have the groom with them, they are not able to fast.
20But the days will arrive when the groom will be taken away from them, and then they shall fast, in those days.
The question the period asked, and the answer that moves fasting from obligation to occasion. The image of the groom indicates that the practice follows the moment being lived, not a marked date — and it explicitly foresees that there will be a time to fast.
2Now as they were ministering for the Lord and fasting, the Holy Spirit said to them: “Separate Saul and Barnabas for me, for the work for which I have selected them.”
3Then, fasting and praying and imposing their hands upon them, they sent them away.
Communal fasting tied to a decision, not to repentance. It is the passage that shows the practice working inside the ordinary life of the early church, associated with discernment and with sending people out.
12Now, therefore, the Lord says: “Be converted to me with your whole heart, in fasting and weeping and mourning.”
13And rend your hearts, and not your garments, and convert to the Lord your God. For he is gracious and merciful, patient and full of compassion, and steadfast despite ill will.
The best-known call to collective fasting in the Old Testament, with a caveat built in: the next instruction sets rending the heart against rending the garments. The outward gesture on its own is refused in the same breath in which the practice is asked for.
6Is not this, instead, the kind of fast that I have chosen? Release the constraints of impiety; relieve the burdens that oppress; freely forgive those who are broken; and break apart every burden.
7Break your bread with the hungry, and lead the destitute and the homeless into your house. When you see someone naked, cover him, and do not despise your own flesh.
The assessment criterion. The text does not discard fasting: it redefines what counts as an accepted fast, in terms of releasing the constrained, sharing food and sheltering those without a house. Anyone wanting to measure their own fast has the biblical yardstick here.