Answer:
Mark 11:26 is not in the NIV or any of the best modern translations of the Bible, because it's clear to translators that some scribe added that phrase possibly hundreds of years after the original writing by Mark. So modern translations take those verses out and put them in the footnotes, leaving a gap in the verse numbering. But you might ask, how did those additions get in there in the first place?
Well, the NT enjoys more textual support than ANY ancient document. The down side of having thousands and thousands of NT copies is that there?s bound to be variant readings. Some texts were not copied as accurately as others. In the ages when the Bible was being copied and transmitted by hand, there was no printing press and no internet communication and little travel to help crosscheck the work.
So some of these additions got passed on and some even became associated with certain geography. So for example, the Western church went with one variant reading of a passage and the Eastern church another. Eventually some of these late additions found their way into many manuscripts and that's how they landed in translations and got put in the standardized verse numbering (which was added in the 14th century).
Now, many of these copying mistakes did not start as mistakes at all. Early Christian scribes knew the Bible amazingly well (some had the NT memorized!) and so they would often write in the margins cross referencing other verses. So when a story in Mark was similar to a story in Matthew, but Matthew had additional info, the scribe would quote the Matthew verse on the side. The next scribe might insert that cross reference into the text to help Harmonize the different gospels into one. The next scribe would keep the cross referenced verse in the body of the text, and finally after two or three reproductions, the added verse is treated as if it were Mark's verse, when really it was Matthew's.
This is exactly what Mark 11:26 is. It is not Mark?s work. It is Matthew 6:15 which says, "But if you refuse to forgive others, your Father will not forgive your sins." A scribe put it in there to help readers harmonize all the gospels better and in some cases, where people didn't have Matthew, but had Mark, this would be a way of getting them a broader gospel, call it Mark with bonus material!!
Some of these variant readings are not so minor. Like the whole ending of Mark (16:9-10). It's a late addition built the exact same way as above. You can see in Mark 16:9-20 quotations from the great commission of Matthew 28, the assencion of Luke 24:51 and the reference to snake poison from Acts 28:5. So it's not "bad" stuff, or invented out of whole cloth... in fact, most of these variants are simply Scripture inserted from other places. Which is why Mark's longer ending is still included in many Bibles.
But in the interest of honesty and integrity, Bible's today note that Mark did not write 9-20 - someone else did. And you can see pretty easily why someone did. Mark's work is so fresh and amazingly first hand, that it sort of ends leaving you hanging (Mark 16:8) - the witnesses to the resurrection are left in total fear and bewilderment. It's fresh and raw, showing it's accurate as history, but as a story, you want to know more, you want more of a bow. So later Christians added one.
How can you tell which are late additions? The easiest way is to look at manuscripts. If only the late manuscripts have a given phrase, it's probably been added.
It?s important to understand that the variants don't change any core Christian teaching at all. As I said, most of these are just Scripture inserted from other places. However, some insertions are scribes making elaborations to clarify accepted theology. So one that is like that (which you still find in the KJV) was accepted because it affirms the Trinity very clearly: I John 5:8: "three that testify in heaven: the Father, The Word and the Holy Spirit and these three are one..." But that verse is now put in the footnotes in modern translations because again this is a late addition not found in the oldest manuscripts... in fact this verse is not found in any manuscript older than the 14th century... so it is VERY late.
When it was added, the Trinity was universally accepted doctrine and the verse paralleled the original phrase in 1 John very nicely and it was poetic. So it gained quick acceptance. But again you can see that in this case, when it's TOO neat, TOO clear, TOO tidy, it's a strike against authenticity. In the 1st century, when the NT was being written, the Trinity was an unknown phrase and the doctrine undefined. Of course, the truths that formed the doctrine (monotheism, and the divinity and personhood of the Father, Son and Holy Spirit) were there in the apostles minds and writings, but the full understanding of God's Triune nature was not fleshed out so succinctly until the 3rd & 4th centuries.
Another variant reading that is most likely late and not authentic, is a passage that also doesn't affect theology but it is a sentimental favorite. It's John 7:53 to 8:11. It bothers many people to find out that John probably did not write the story of the woman caught in adultery, but again, no core teaching or truth is at stake. Jesus' compassion for sinners is fully documented elsewhere, but this particular passage was not written by John and so there's no way to tell if it's historically accurate. It may be, but it doesn't carry the authority of John behind it, because it was written after John. The story carries the same themes as all the gospels and exudes the same spirit of grace in Jesus as we see elsewhere, and so makes for a good teaching tool as a story passed around the early church... but it should not be read on the same level as other Scripture.
