Matt 17:21 (TLB) but this kind of demon won’t leave unless you have prayed and gone without food.

This verse 21, that we find only in the Gospel of Matthew, is not a part of the original texts.
Few Christians know it because it is rarely taught, and even less explained that it is not a part of the original text.
This verse 21 was placed here for a better understanding.

Except that... The problem is that some translators have added their own thinking.

If we look up a few verses earlier, we can go back to verse 15.

Matt 17: 15-18 “Lord, have mercy on my son, for he is an epileptic and suffers severely; for he often falls into the fire and often into the water. So I brought him to Your disciples, but they could not cure him.” Then Jesus answered and said, “O faithless and perverse generation, how long shall I be with you? How long shall I bear with you? Bring him here to Me.” And Jesus rebuked the demon, and it came out of him; and the child was cured from that very hour.

All the exchange between Jesus and His disciples was about the subject of unbelief. It is indeed because of unbelief or lack of faith that the disciples could not heal this child. In fact nobody talked about demon, even not his father, because he said my son is moody, (or epileptic in the original).
Some translators have thought good to add the verse 21 speaking about demon or evil spirit, but it has never been question of this in the conversation between Jesus with His disciples.

So the majority of Christians believe there are demons for whom fast is needed, because they are so strong that we cannot cast them away by simply praying with the authority that we have received in Christ.
This thought has been conveyed by the devil himself, and resumed by ignorance by many pastors and various leaders. That maintains Christians in a position of weakness regarding some demons. Most of them did it thinking doing well. We don’t condemn anyone

More than 8 versions of the Bible have removed this verse 21, or wrote: "this kind..." implying that kind of unbelief won’t leave except by fasting and prayer.
The Interlinear Bible also confirms this, as verse 21 is not in the original.

The truth is that this passage is not a part of the original Scriptures. It is therefore not to be considered as inspired Scripture.
My point is not to denigrate the Bible, nor tell you to put it in doubt, but simply to study it more deeply.

Then you won't be fooled by satan and his lies.
He is defeated, made a public spectacle of him, he flees before us, but to see that coming alive, we must believe with all our hearts without doubting. That's what Jesus said to His disciples in this passage from Matt 17, and nothing more.
So, there has never been question about demons being cast out only by fasting, this is in total contradiction with the rest of the teachings of the new Covenant.

Bye for now…