Let's take a moment to look at two of the three largest, most dramatic, most sensational instances of the 40 variations that actually exist. (I'll look at the third next time.) Conveniently Ehrman spends quite a bit of space and ink relishing in these three because they are the most sensational instances to which a person can possibly point in an attempt to undermine a person's confidence in the reliability of the text of Scripture. I'll briefly address them in their canonical order.
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Mark 16:9-20.
There is debate as to whether Mark 16:9-20 is part of the original Gospel of Mark. Here is a basic rundown of the facts. All but two Greek manuscripts have verses 9-20 in them. One of those two looks as though verses 9-20 were written down and then someone erased them at a later date. There are a few later manuscripts in other languages which do not have verses 9-20. There are also a few manuscripts which place a small two sentence summary of verses 9-20 after verse 8, in place of verses 9-20. Finally a few manuscripts record all of verses 9-20 before tacking on the two sentence summary of them.
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Regardless of whether you decide Mark 16:9-20 is original or not, I will simply point out one fact: unless you are trying to prove that all Christians should drink poison and handle snakes as proof of their spiritual authenticity, there is no doctrine gained or lost in this passage. (And even if you were attempting to prove this belief, Mark 16:9-20 would far from make your case.) The same basic information in the last part of Mark 16 is recorded from slightly different perspectives in the other Gospels and in the first chapter of Acts. And chances are that this passage will be in brackets with a footnote in any Bible you pick up. This is not exactly the ancient, sensational Christian conspiracy Ehrman would like to make it out to be.
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John 7:53-8:12.
The woman caught in adultery is a favorite story among many believers. People love to quote the line, “He who has no sin throw the first stone.” We love to guess as to what Jesus started writing in the dirt as he knelt down. Somehow this story seems so similar to what we see elsewhere of Jesus, that it is hard to believe it may not be part of the original text. However, its position as original is highly in doubt. In summary, this story is missing in many manuscripts. In some manuscripts it is present, but with an asterisk in the margin before and after it, not too unlike the brackets and footnote you will likely find in your Bible translation if you turn to this passage. Other manuscripts place it at the very end of John, tacked on as an appendix that doesn't fit within but is too good to leave out. And to make matters more interesting this story even appears in a few manuscripts of the Gospel of Luke.
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It seems highly unlikely that this story is a part of the original Gospel of John. Still yet, it rings so true to what else we see of Jesus in John that it is hard to let the story go. My personal opinion is that it is a true story of Jesus that was passed on by someone who was copying the Gospel of John. They included it as an appendix or an additional story, not unlike the bonus track on a modern CD. Some later scribes thought that it was a part of the Gospel that was unintentionally left out and added as an appendix. They then included it in the body of the Gospel and caused confusion for later scribes.
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However, I must point out that whether this story is original or not, it does not add or take away from what we see of Jesus' character and teachings. Part of the reason it is so well loved by many is that it exemplifies the compassion, mercy, and teaching Jesus demonstrates elsewhere in John and in the other Gospels. Like the passage in Mark, this gains shock value for people who have not read their footnotes, but it does not actually build Ehrman's case that the validity of orthodox Christianity or the truth of Jesus' teachings are in doubt.