I have written before about textual criticism, which refers (at least in biblical studies) to the study and comparison of biblical manuscripts in order to give us a more accurate picture of what the original documents said. A lot of Christians are largely unaware of this field of study, and only become aware of it when they see footnotes in their Bibles near certain passages that say something like, “many of the earliest and best manuscripts do not contain these verses.”
It can be alarming for some people when they read footnotes like these because it seems to throw doubt over whether or not we can trust our modern Bibles. Really though, the opposite is true: it is only because we have such a wealth of New Testament manuscripts that we are even aware of the discrepancies between different ones:
We don’t have the original editions of the Bible. Instead, what we have are thousands and thousands of handwritten copies called manuscripts. We have fragments that date back to the early second century, but the best comprehensive manuscripts we have that contain most or all of the New Testament date back to the fourth and fifth centuries.
Compared to other ancient works, this is incredible. There are some ancient works of famous philosophers or poets of which we may only have a handful of copies, but there are thousands and thousands of biblical manuscripts. There are a lot of differences between the different manuscripts because they were copied down by hand, but since there are so many copies, we can compare them and, with a very high degree of accuracy, determine what the original text said.
The vast majority of differences between manuscripts are differences in things like spelling (basically the modern equivalent of a typo) where it is still very obvious what is supposed to be said. There are only a handful of places in the New Testament where there is a whole verse or verses that we are not sure about, and even in those, there is no point of doctrine that is compromised either way. So the biblical text that we have is very reliable.
John 7.53-8.11, the story of the woman caught in adultery, is perhaps the most famous textual problem in the New Testament, but another is Mark 16.9-20, sometimes called “The Long Ending of Mark.” It reads:
9 [[Now when he rose early on the first day of the week, he appeared first to Mary Magdalene, from whom he had cast out seven demons. 10 She went and told those who had been with him, as they mourned and wept. 11 But when they heard that he was alive and had been seen by her, they would not believe it.
12 After these things he appeared in another form to two of them, as they were walking into the country. 13 And they went back and told the rest, but they did not believe them.
14 Afterward he appeared to the eleven themselves as they were reclining at table, and he rebuked them for their unbelief and hardness of heart, because they had not believed those who saw him after he had risen. 15 And he said to them, “Go into all the world and proclaim the gospel to the whole creation. 16 Whoever believes and is baptized will be saved, but whoever does not believe will be condemned. 17 And these signs will accompany those who believe: in my name they will cast out demons; they will speak in new tongues; 18 they will pick up serpents with their hands; and if they drink any deadly poison, it will not hurt them; they will lay their hands on the sick, and they will recover.”
19 So then the Lord Jesus, after he had spoken to them, was taken up into heaven and sat down at the right hand of God. 20 And they went out and preached everywhere, while the Lord worked with them and confirmed the message by accompanying signs.]]
In most modern translations, these verses will either be included in brackets (as the ESV does, which I have tried to preserve above) or will be omitted from the main text and perhaps included in a footnote. This is not because some sinister forces are seeking to alter the content and meaning of Scripture from what was originally written; rather, it is a reflection of the text-critical belief that these verses were not originally part of Mark, based on the fact that many of the earliest and best manuscripts that we have of Mark’s Gospel do not contain them.
Ultimately, biblical scholars disagree about the authenticity of the long ending of Mark. Most hold that it is not original, but those scholars who believe it to not be original are also divided about whether or not there was a different original ending that has been lost, or if the original version of Mark’s gospel was intended to end after verse 8.
I am undecided myself: I tend to think that the long ending is not original and that Mark wrote his gospel to conclude at 16.8, but I could certainly be mistaken. Either way, here is the important idea (and, indeed, the important idea to keep in mind with all of the text-critical issues in the New Testament): there is no doctrine or practice discussed in Mark 16.9-20 that is not taught elsewhere in the New Testament. In other words, even if you throw out all of the passages with significant text-critical problems, it doesn’t change Christian faith and practice.
As one commentator states:
Our God has not seen fit to exempt the New Testament from the copying problems that existed in all books prior to the invention of the printing press. But by his grace those problems do not create significant variations in Christian beliefs and practices.
If we only had one manuscript copy of the New Testament, we would have no variations. That sounds nice, but really, it would leave us with no way of knowing how accurate our Bibles are. Instead, the thousands of manuscripts with their many variations help us to determine with a high degree of accuracy what the original text said, and what it is that God wants us to know.
What a blessing—may God be praised for His faithfulness in the preservation of his revealed word!
 Excerpted from Pardon, Not Acquittal: Jesus and the Woman Caught in Adultery.
 Allen Black, Mark, The College Press NIV Commentary (Joplin, MO: 1995), 293 note 2.