Hi IndyEllis,
Let me see if I can take a stab at this one.
The claim to Biblical inerrancy applies to the original written manuscripts, not to the copies or translations that we have today. With very few exceptions, most Bible scholars will readily admit that variations have crept into the various families of source texts upon which our translations are based.
The more conservative scholars trust that the documents are very faithful to the originals, but few would claim that they are 100% faithful.
When Bible translators consider a passage such as Matthew 17:20-21, they find some texts including v21 and some which do not. At this point they have to make a decision as to whether to include it in their translation based upon the age of the documents considered and the perceived reliability of the documents. Often there is intense debate amongst them as to how to deal with these verses.
In this case, it appears that the NIV translators felt that the most reliable documents did not contain v21. However, I applaud them for including it as a footnote so that the reader knows that it was included in at least some of the documents.
I'd like to also note that whether or not this verse is included in the translation has little bearing on the power of prayer and fasting. The Bible discusses this issue in many other places, and this passage is consistent with the Bible's overall teaching regardless whether or not v21 is included.
Well, that's my take on it anyway. Have a great day
