Mk 9:29 – And he said to them, “This kind cannot be driven out by anything but prayer and fasting.”
Should the “and fasting” be in or out of the text?
There is a minority of evidence that points to the exclusion of “and fasting,” but it is from B (Vaticanus) which is a very trustworthy source.
I think this type of variant must be weighed against the age of the manuscript as Church history can give us a clue here. The early Church held fasting as being very important. Any evidence that may be of a later date that included the phrase could be because it was added to the text in order to give more validity to a practice that was so common to the Church.
However, personally i would be very reluctant on that ground alone to put so much weight on B (Vaticanus) while rejecting so many other witnesses. Witnesses including the phrase – A (Alexandrinus), C (Codex Ephraemi), D (Codex Bezae), as well as many others.