While most Reformed scholars use and endorse the Critical Text in academic and professional settings, they sometimes “revert back” to the Received Text while quoting the Bible from memory.
This does not necessarily prove anything, but it is definitely good to see. Maybe it will get people wondering, “Why did he just quote a verse that does not appear in my Bible?”
Below are some resources on the verse in question. If you have a resource worthy to be shared, please add it to the comments.
Does it matter …? Does Matthew 17.21 belong in your Bible? [Trinitarian Bible Society]
Mark 9:29 and Prayer and Fasting and the Early Church [Text of the Gospels]
Mark 9:29 and Fasting – More External Evidence [Text of the Gospels]
Matthew 17:21 – What’s the Early Evidence Say? [Text of the Gospels]
Mark 9:29, Prayer and Fasting, and Some Early External Evidence [Text of the Gospels]
Mark 9:29 – Conclusion: “and fasting” is Original [Text of the Gospels]
Demons LOVE how the Critical Text omits FASTING from Mark 9:29 [Dwayne Green]
The CRITICAL TEXT removes “FASTING” when casting out demons [Dwayne Green]