Jesus mentions fasting, in the sermon on the mount, in Matthew, chapter 6: He says, “and when you fast..." Jesus gives instruction to his disciples on how to fast differently than the super-religious people of the day. The purpose of fasting, like all kingdom activity, is to bring our lives into alignment with heaven. God rules and reigns in heaven, without restriction. In fasting, we seek God's rule and reign in our whole being.
I got the idea that we can fast from the world to find the kingdom of God. This was perhaps the original idea behind monasteries and hermitages. This is also the idea behind getting away to pray and retreats. Jesus himself went away to pray.
But the majority of the majority of Christ-follower's lives are lived in the world. For the Christian, worldly appetites need to be abstained from, to make way for the kingdom in our lives.
In fasting, we deny our selves worldly things and instead focus on spiritual food. In fasting, we train our appetites that they come second to our desire for God.
The values of the world are at odds with God's values. Everywhere, the world is involved in rebellion against God: Idolatry, deception, and lies. The Christian lives in the midst of this. How then do we function in our new walk outside the world, while in the world? By intentionally fasting from the world as a way of life. The disciple of Jesus lives a life of God first, all the time, everywhere.
This is sounding hard, you might say. But, God gives us grace, more grace (James 4:6) to work out walking with him and under the banner of the kingdom in this world. God gives us latitude and leeway in our walks. In the new covenant it is all about Jesus. He has done it all already. You cannot earn anything, but you get to decide, every day, how far you want to go. You really can decide to be cold or lukewarm or hot, and if you are a son or daughter, you can expect discipline, with love, from God. We are not under law, but as sons and daughters, the bar is actually higher.
In the sermon on the mount, after Jesus mentions fasting, everything he says, up to verse 34, is instructive on how to fast from the world. Remember that for everything that Jesus instructs, God has already made provision for you to do, with the help of the Helper, the Holy Spirit.
"Don’t put on a sad face like the hypocrites." You may feel sad and get sad and therefore, look sad. That is ok. Jesus is saying to not put on the sad mask, to be a spiritual phony. Keep is real, be genuine, be yourself.
Jesus contrasted his ways with the religious leaders of his day. Hypocrisy was their corporate culture: saying one thing and doing another. The religious hypocrite has many masks. They are fake actors, rather than real, transparent, and authentic.
Don't be like the religious hypocrites Jesus says. They mouth things about God, but they are man centered. They are law centered, devoid of the Helper, the Holy Spirit.
"You won’t look like you are fasting to people, but only to your Father who is present in that secret place. Your Father who sees in secret will reward you." Fasting is something you just do in the kingdom. You do not have to do it, and your are not required to do it. You might fast and pray if you feel so led, or you might fast (stop) something in your life that is impairing the kingdom in your life. You won't need to advertise it. Christ followers have a secret life with God.
One of the marks of a Pharisee is that they don't do things in secret, but display them. The Pharisee also gets a thrill out of the display being seen, of showing and telling. Each Follower of Jesus has their own secret history with God, which God rewards. You don't need to show off.