As footnotes in Revised Standard Version and Good News Translation both indicate, some manuscripts add verse 21. It is the consensus of scholars that this verse was added to the Gospel at a later date, perhaps under the influence of the parallel in Mark 9.29. There are no apparent reasons why a scribe would have removed the verse, if it had been an original part of the text.

This kind … comes out … by prayer and fasting is semantically equivalent to “only prayer and fasting can drive this kind out” (Good News Translation). For some languages it may be necessary to translate the two nouns prayer and fasting as verbs: “Only if you pray and fast will you be able to drive this kind out” or “You can drive this kind out only as you pray and fast.” Since this kind is a reference to demons, Die Bibel im heutigen Deutsch translates “… can such spirits be driven out.”

For a discussion of fasting, see comments at 6.16. The sentence can be “You can only drive out this kind of evil spirit by praying and worshiping God by not eating” or “This kind of evil spirit can only be driven out of a person when people have prayed and worshiped God by going without food.”

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on the Gospel of Matthew. (UBS Handbook Series). New York: UBS, 1988. For this and other handbooks for translators see here .