If you turn to Mark chapter 16 in a modern Bible translation, you will probably find a note somewhere that says that certain "early and important" manuscripts do not contain verses 9 through 20. A few translations also have a short paragraph of text that is not assigned a verse number. Should these verses be part of the Bible, if they're missing from these manuscripts?
Before answering this question, I'd first like to emphasize that this is not a liberal/conservative issue. Those who believe that the Bible is merely a human book don't really have much basis for including or excluding verses from the Bible other than tradition. But for evangelicals who believe the Bible is inspired, it is important to know that the text we see on the page is a good translation of words inspired by God. When it comes to the Gospels, we believe that what was inspired is what the author included in the released form of his book.
Manuscripts differ from one another because they were copied by hand, and the copyists made some mistakes or intentional changes along the way. Where they differ, we have to compare these manuscripts with one another to determine which reading matches what the author originally wrote. This task is called textual criticism. In most cases it is relatively easy, and evangelical Bible scholars are basically in agreement for most of these variations.
There are a number of things that make the Mark 16 issue quite difficult. First is the length of the reading in question. Most textual variants involve the order or form of one to four words, or small insertions, omissions, or substitutions. There are a number of places in the Gospels where an entire verse is in question. But only here and in John 7:53-8:11 is an entire passage present in some texts and absent in others. It seems unlikely that anyone would insert such a large section to the Bible, and equally unlikely that they would and could remove it.
The second major problem is that the passage is absent in two of the most important manuscripts but present almost everywhere else. Thus both the presence and absence of the passage have impressive support.
Third, quotations from early Christians indicate that the passage was known very early on, but also that it was absent in some manuscripts very early. Both situations predate the manuscripts that exist today.
Let's take a close look at the evidence for the different endings we see:
1. A few witnesses indicate that Mark 16:1-8 ends the book.
Two manuscripts known as Sinaiticus and Vaticanus have this ending. They are the earliest copies of the entire New Testament, and both date from the 300s. They represent two different branches of what we call the Alexandrian type of texts. Most of the very early papyri and other evidence usually supports readings found in the Alexandrian texts as opposed to those of other text types. It is true that Vaticanus has a blank column after Mark 16, but it is not quite long enough to contain the passage. It is possible that the scribe knew of verses 9-12 and wished to indicate their existence or make room for them, but either did not believe them original or did not find them in his copy.
The Sinaitic Syriac version, which also dates from around 300 and has connections with the Greek codex Sinaiticus, also ends with 16:8.
A variety of other versions end with verse 8: one in Sahidic, the Adysh and Opiza manuscripts of Old Georgian, and many Armenian manuscripts. This indicates that this version of Mark was rather widespread. (One resource I've seen also cites some Old Slavonic manuscripts from the 900s that end with verse 11, but they are not listed in most of my sources.)
One late Greek manuscript, Minuscule 304, dates from the 1100s and lacks this text. It is the only surviving Medieval Greek witness to this ending.
Early church fathers such as Clement of Alexandria, Origen, Cyprian, and Ammonius, cite Mark often but make no references to 16:9-20.
Eusebius (c. 340) created the Eusebian Canons, a numbering system to harmonize the four Gospels. These canons do not make room for Mark 16:9-20, even though the passage makes reference to events in the other Gospels. I've read that Eusebius also makes mention of the fact that some manuscripts of his day contained verses 9-20 but some did not.
Jerome said of verses 9-20 that "Almost all the Greek copies do not have this concluding portion" (Epist. css.3, ad Hedibiam, c. 420). In other words, although the majority of manuscripts in our day have verses 9-20, the majority in his day did not. Jerome nevertheless included the verses in his Latin Vulgate.
2. One witness has an insertion in verse 3 and the "shorter ending."
The earliest and most important Latin manuscript is Bobbiensis (often called k). It is an African text that goes all the way back to about A.D. 200. While k is generally reliable, it adds a description of Jesus' resurrection to the third verse: "But suddenly at the third hour of the day there was darkness over the whole circle of the earth, and angells descended from the heavens, and as He was rising in the glory of the living God, at the same time they ascended with him; and immediately it was light. Then the women went to the tomb...." After verse 8, there is a short paragraph summarizing the commission of the disciples.
3. Quite a few manuscripts follow 16:8 with the shorter ending, and then add verses 9-20.
The Harclean Syriac version of Mark, going back to about A.D. 515, contains both the shorter ending of k and the longer ending we recognize as verses 9-12. The shorter ending is in the margin.
The oldest Greek witnesses containing both endings are an Alexandrian codex called Regius (L) and two manuscripts numbered 083 and 099. They date from around A.D. 700. Uncial 099 is interesting in that it is a small scrap containing only Mark 16:6-20 and the shorter ending.
From the 900s we have the beautiful vellum manuscript Athous Laurae, which is a mix of Alexandrian and Western text-types. Also from this time is Minuscule 274, which places the shorter ending in the margin.
The latest Greek manuscript containing both endings is Minuscule 579, an Alexandrian manuscript from the 1200s.
Early versions in other languages that contain both endings are some Bohairic and Ethiopic manuscripts, and all Sahidic manuscripts except the one that stops at verse 8 (cited above).
4. Some manuscripts follow 16:8 with verses 9:20 and add additional material between verses 14 and 15.
Jerome spoke of several manuscripts that contained an expanded form of the longer ending. After Jesus rebukes the disciples for their unbelief, "They excused themselves, saying, 'This age of lawlessness and unbelief is under Satan, who does not allow the truth and power of God to prevail over the unclean things of the spirits. Therefore reveal your righteousness now.' Thus they spoke to Christ. And Christ replied to them, "The term of years of Satan's power has been fulfilled, but other terrible things draw near. And for those who have sinned I was handed over to death, that they may return to the truth and sin no more, in order that they may inherit the spiritual and incorruptible glory of righteousness that is in heaven.'"
Jerome was the only witness for this ending until Codex Washingtoniensis (W) was discovered by Charles L. Freer in 1906. The manuscript is a wildly mixed text dating from the 400s, and the expansion is known as the Freer Logion. With just one extant text and Jerome's citation, it is impossible to know how widespread this reading was.
5. Quite a few manuscripts follow 16:8 with verses 9-20, but indicate that the verses are in doubt.
Most prominent among these are what we call the "family 1" manuscripts. They set off the closing verses with obelisks, a scribal convention used to indicate text that was spurious or questionable but nevertheless present in the scribe's source. The earliest of these texts dates from 948. They include Minuscules 1, 118, 131, 209, and 1582, and are usually considered to represent what we call the Caesarean text-type.
Most of the sources I use, including the Nestle-Aland Greek New Testament, include "others" in the obelisk-marginal-note list but do not specify which or how many other manuscripts follow this pattern. Please e-mail me if you know of others. (Update: Ms. 1, 15, 22, 205, 209, 1110, 1192, 1210, and 1582 all contain the same note introducing verses 9-20: "In some of the copies, the evangelist is completed here, and here is Eusebius Phamphili's canonization. But in many, this also exists." These identical notes likely trace back to a single source. Thanks to Jim Snapp for this information and other corrections.)
6. The remaining manuscripts follow 16:8 with verses 9-20 without comment.
Significant early Greek witnesses here include the Byzantine codices Alexandrinus and Ephraemi, the Western codex Bezae, and the Caesarean codex Koridethi.
Early versions in other languages include most Old Latin manuscripts, the Latin Vulgate, the Curetonian Syriac (300s), the Syriac Peshitta (400s), and some Bohairic manuscripts.
The famly 13 manuscripts (13, 69, 124, etc.), Minuscule 33, and Minuscule 2427, and the majority of Greek manuscripts from 895 onward have just this longer ending.
Tatian's Diatessaron, an early 2nd-century harmonization of the Gospels, evidently included this ending.
Justin Martyr and other early Christians have strong verbal parallels (i.e., possible quotes) with verses 9-20, as does the Latin translation of Irenaus (translated c. 395).
Primary theories of Mark's ending
All the extant additions to 16:1-8 are attempts by scribes to account for the apparent lack of completion to the book.
1a. Because Mark intentionally ended the Gospel with verse 8.
1b. Because Mark died without completing his Gospel. Or...
1c. Because Mark's original ending was lost.
Verses 9-20 are original, and the shorter ending is an attempt by scribes to fill the gap where verses 9-20 were somehow unknown.
2a. Because Mark released one edition ending with verse 8, and one ending with verse 20.
2b. Because an unknown (but inspired) apostle added verses 9-20 after Mark released his Gospel.
2c. Because certain copies lost their final page.
2d. Because a number of scribes accidentally stopped at verse 8. Or...
2e. Because a number of scribes intentionally removed verses 9-20.
2e-1. Because they thought it was a spurious addition.
2e-2. Because they had doctrinal problems with the passage. Or...
2e-3. Because they were evil and wanted to corrupt the Bible.
My (Somewhat Technical) Take
The question of the ending of Mark's gospel revolves around developments that preceded any of the extant manuscripts or citations. As far back as the 200s at least, there were some manuscripts ending at verse 8 and some at verse 20. The history of transmission from that point onward makes sense. Especially with such a long passage, it may be better to include something spurious that can always be disregarded upon future evidence, than to allow a possibly original reading to pass into obscurity. (This is why, in my opinion, the most important feature of a Greek New Tesatment is its textual apparatus listing variant readings. This way the editors leave their decisions open to testing.) The testimony of Sinaiticus and Vaticanus are not determinative here, for this is a unique problem that can't simply be solved by appeal to a particular text-type. Nor is internal evidence very helpful. Arguments based on vocabulary, grammar, and interpretive issues are hard to establish objectively. The issue is one of probability based on the evidence we have: was the passage more likely to be omitted or added?
The apparent absence of such a long passage over such a wide area of indicated by the evidence, along with the fact that the verse-8 ending was apparently the majority reading known to Jerome, suggest to me that the passage is not original. If the passage were omitted, the deletion must have happened more than once, or else the scribes (who knew the Bible well) would certainly have caught the mistake and either sought out another manuscript or made a note that text was missing. Nor does it seem likely to me that some conspiracy is responsible for removing these verses. On the other hand, if I had to compose an ending for Mark, given 16:1-8, I would probably do much was the author of 9-20 did--summarize post-resurrection appearances from the other Gospels and include something similar to the Great Commission.
As for the content of the passage, there is no doctrine or story here that does not appear elsewhere; the passage is not "crucial" in that sense. However, there are some interpretive pitfalls--such as Jesus appearing "in another form," the baptism issue, and the signs of verses 17 and 18--that have led some to feel that this passage doesn't meet the standard of consistency with the rest of Scripture. But all God's revelation is important, and there remains enough possibility that the text is inspired (perhaps added by an apostle after the Gospel began circulating) that I think Bible versions should include it. However, I also think a footnote or other indication should make the reader aware that there is at least some question as to its originality.