A careful study of the synoptic gospels clearly reveals that Matthew and Luke made use of Mark when they compiled their gospels. Therefore the three gospels correspond nearly word for word in a huge amount of material. But Matthew and Luke had another source called “Q” (Quelle, German for a well) in which they both correspond, but is not found in Mark. And then each of them also did their own unique research, adding material that is unique to that gospel only.
In Mark 6:11 we are confronted with a variation where the clause in uppercase (KJV) is not found in the modern Bibles like the NIV.
“And whoever shall not receive you, nor hear you, when you depart from there, shake off the dust under your feet for a testimony against them. TRULY I SAY TO YOU, IT SHALL BE MORE TOLERABLE FOR SODOM AND GOMORRAH IN THE DAY OF JUDGEMENT THAN FOR THAT CITY.”
If we take the relationship of the three gospels into account, there must be two possible causes for this variation.
1) If this clause had been part of the autograph of Mark, one would have to find an answer as to why Luke did not include it into his gospel. It is unthinkable that Luke would for some reason discard such a definite warning, if indeed it had been part of the original. There are also no manuscripts at all that do include this clause in Luke 9:5 where the rest of this incident is recorded. That leaves us with no other conclusion that it had not been part of Mark when Luke utilised Mark when he compiled his gospel.
2) The other possibility is that it had not been part of Mark in the first place, but had in fact been part of Matthew’s unique research, recorded in Matthew 10:15. From there it must have been interpolated into Mark. This seems to be the real explanation for this variation for the oldest manuscripts are without this clause. The oldest Greek codex including this clause is Basiliensis of the 8th century, but then in most of the later codices and mediaeval manuscripts.
There is no explanation possible why all Greek manuscripts prior to the 8th century are in fact without this clause other that it had not been part of the original autograph.
We have to conclude that this clause is part of Matthew’s unique research and had been interpolated into Mark. Removing it from Mark is restoring Mark to its original autograph as God had inspired Mark.
God bless,
Herman.
Your commentary is very welcome at the bottom of this page, or to me at bibledifferences@gmail.com
Bible differences 
Hi Herman,
Good analysis, I totally concur on the removal of the clause from Mark as restoration of originality. I am doing some comparative synoptic account studies and Matthias (Matthew) was a probable recipient of the teaching as he often includes various additions not in Mark or Luke. The clause itself is possibly an allusion to the well-known admonitions of the O.T. idea of “the double sin.” That Israel was privileged by knowing God and to depart from Him was one offense and to serve other gods another. It is the same idea for Christians today: to whom much is given, much will be required.