Why does the New International Version of the Bible leave out some verses?

Question:

Why does the New International Version (NIV) of the Bible leave out some verses, such as Matthew 18:11?

Answer:

There are several thousand manuscripts of the Greek New Testament dating back as early as the second century A.D.  These display only very minor variations, no doubt due to copying errors.  Some of the manuscripts include Matthew 18:11 while others omit this verse.  Biblical scholars attempt to classify these manuscripts into “family trees” according to the variations found in them so as to get as close as possible to the original writings of the New Testament.  But disagreement remains among scholars as to which manuscripts are closest to being the originals.