Some scholars believe that the ending of Mark is unoriginal due to it appearing to have a different style and the fact that it's missing from some key manuscripts. At the same time, several early Church fathers are believed to have quoted from these passages. What are the primary arguments for and against its authenticity (along with sources, please)?
My understanding is that a strong majority of scholars (including conservative scholars) take the position that the long ending of Mark was not in the original and was not written by the same author as the rest of the text, but nonetheless was added very early on (probably in the early 2nd century). However, the evidence is not as overwhelming as for the Comma Johanneum or the Pericope Adulterae, in part because the long ending of Mark is significantly older than the those two.
The main arguments are as follows. Certainly people dispute some of these arguments, but on the whole each of them is a strong argument, and taking several together gives an even stronger argument.
- The long ending does not appear in several of our earliest and best manuscripts, most notably Sinaiticus and Vaticanus (although it does appear in Alexandrinus).
- Many early manuscripts which do contain the long ending nonetheless contain indications marking it as disputed.
- The existence of manuscripts containing a different ending entirely (the "short ending") also suggests that the original contained no ending.
- The author of Mark has a distinctive Greek style, and the long ending does not match this style.
- The author of the long ending appears to be familiar with possibly Matthew, probably Luke, and possibly Acts, while the author of Mark was not.
- The authors of Matthew and Luke do not appear to have had the long ending in their copies of Mark.
The textual evidence, which covers the first three arguments, as found in Nestle-Aland is summarized ably at the end of the Wikipedia article under "Summary of manuscript evidence." At any rate all of these are largely undisputed facts, though there are some interesting features of that part of Mark in Sinaiticus and Vaticanus which are worth noting (explained thoroughly with images, though also with a bit of an agenda, here).
A quick summary of the argument from style and vocabulary is given by Bruce Metzger in A Textual Commentary on the Greek New Testament excerpted online here. A much more in depth examination is given in a paper of Travis Williams.
For the last point, you want to first notice many similarities between the long ending and Luke/Acts (as well as a small overlap with the great commission in Matthew), and then you have the trickier point of arguing that it's the long ending taking from Luke and not vice-versa. Wikipedia lists the overlaps (I haven't found a good scholarly resource for this point, though of course you can compare the passages yourself). I haven't found a good reference for arguing in which direction the borrowing is going, but if you want to argue the other way you'd need to explain why Matthew drops almost all of it (despite containing 94% of Mark) and why Luke substantially rearranges it (despite usually following Mark's order reasonably well).
It's worth noting that unlike with say the Comma Johanneum, I don't think it's been conclusively proved that the long ending of Mark is not original. Assuming it was added, it was added earlier than any copies of the text that we still have! All sorts of things are unlikely but possible. Nonetheless it seems the evidence is pretty solidly on the side of inauthenticity.
Short Answer: After weighing all of the evidence (both internal and external), it would seem that Mark 16:9-20 was indeed originally part of Mark's Gospel.
The ending of Mark's Gospel is one of the major textual problems in the New Testament.1
As noted by the OP, the "problem" is whether the end of Mark (16:9-20) was originally part of Mark's Gospel. Essentially the matter comes down to three factors.
1. External evidence
Dr. Constable summarizes the external evidence concerning verses 9-20:
The two oldest Greek uncial manuscripts of the New Testament . . . plus many other old manuscripts, do not contain them. Moreover, the writings of some church fathers reflect no knowledge of these verses. On the other hand, verses 9-20 do appear in the majority of the old manuscripts, and other church fathers do refer to them.1
DETAILS AGAINST: absent from א, B, itk, Sinaitic Syriac, ~100 Armenian mss, the two oldest Georgian mss. Some mss that do include it mark it with asterisks or obeli. Clement of Alexandria and Origen show no knowledge of the existence of these verses.3
DETAILS FOR: too many to list!!4... 99% of Greek manuscripts contain it.5 present in the vast number of witnesses, including A C D K W X Δ Θ Π Ψ 099 0112 f 13 28 33 al. Numerous patristic witness beginning with Irenaeus and the Diatessaron.3 Found in many early versions: Vulgate, Syriac, Peshitta, Coptic, Sahidic, Bohairic, Fayyumic, Gothic, Armenian, Georgian, and Slavonic.6
Despite the absence of these verses in some old manuscripts, and the silence on these verses in the teachings of some church fathers (which could be said for any verse in the New Testament), the fact remains that most old manuscripts do contain these verses, and many church fathers do refer to them. Dr. Constable concludes well that:
[these verses] appear to have been regarded as inspired and therefore authoritative early in the history of the church.1
Thus, while external evidence in this case is far from conclusive, the evidence does weigh in favor of these verses originally being part of Mark's Gospel.
2. Internal evidence: structural coherence
Dr. Constable mentions an important structural consideration in this debate:
Throughout this Gospel, we have noted many unique features that appeal to disciples to serve God by bearing bold witness to Jesus, even in spite of persecution and suffering.1
This makes sense in light of Mark's target audience. Mark wrote his Gospel to disciples in Rome who faced a serious decision:
They faced the choice of whether to take a public stand as Christians—and suffer the loss of real estate, personal property, employment, and even their lives—or to lay low. They were required by law to offer a pinch of incense in worship of "divine" Caesar, as Roman citizens. Doing so compromised their exclusive commitment to Jesus as Lord. To fail to worship Caesar cost them dearly.1
Mark's Gospel served as an exhortation to these Roman disciples to lay down their lives and preach the gospel with boldness. In light of this, it is hard to imagine Mark ending his Gospel "with an example of disciples too fearful and amazed to bear witness to the resurrected Jesus."1 Yet this is exactly what we would have if Mark 16:8 were the end of the Gospel. Thus, structural coherence also weighs in favor of Mark 16:9-20 originally being part of Mark's Gospel.
3. Internal evidence: stylistic clues
Bruce Metzger elaborates on the two main considerations:
(a) The vocabulary and style of verses 9-20 are non-Markan. (e.g. απιστεω, βλαπτω, βεβαιοω, επακολουθεω, θεαομαι, μετα ταυτα, πορευομαι, συνεργεω, υστερον are found nowhere else in Mark; and θανασιμον and τοις μετ αυτου γενομενοις, as designations of the disciples, occur only here in the New Testament).
(b) The connection between ver. 8 and verses 9-20 is so awkward that it is difficult to believe that the evangelist intended the section to be a continuation of the Gospel. Thus, the subject of ver. 8 is the women, whereas Jesus is the presumed subject in ver. 9; in ver. 9 Mary Magdalene is identified even though she has been mentioned only a few lines before (15.47 and 16.1); the other women of verses 1-8 are now forgotten; the use of αναστας δε and the position of πρωτον are appropriate at the beginning of a comprehensive narrative, but they are ill-suited in a continuation of verses 1-8.3
Commentator J.R. Dummelow adds a few more:
(1) the true conclusion certainly contained a Galilean appearance (Mark 16:7, cp. 14:28), and this does not.
(2) The style is that of a bare catalogue of facts, and quite unlike St. Mark's usual wealth of graphic detail. . . .
(6) The section seems to represent not a primary tradition, such as Peter's, but quite a secondary one, and in particular to be dependent upon the conclusion of St. Matthew, and upon Luke 24:23f.2
In favor of Mark 16:9-20 being authentic, Scrivener responds to these arguments with the following dismissal:
With regard to the . . . alleged difference in style from the rest of the Gospel, I must say that the same process might be applied — and has been applied — to prove that St. Paul was not the writer of the Pastoral Epistles . . . St. John of the Apocalypse, Isaiah and Zechariah of portions of those prophecies that bear their names. Every one used to literary composition may detect, if he will, such minute variations . . . either in his own writings, or in those of the authors he is most familiar with[!]4
Scrivener is highlighting a major weakness in "stylistic clues": they tend to be easily bent to the will of the interpreter! This has been my experience as well, and so I tend to place less weight on such arguments (though they can be very telling in some cases.) But for example:
Metzger argues that it is strange for Mark to mention Mary Magdalene by name in 16:9 since she had already been named in 16:1, yet note that she is also explicitly named in 15:40 and 15:47 (one verse prior to 16:1)! With all due respect, this is a silly argument, and reflects a haste to justify a preconceived notion.
There are 17 words in Mark 16:9-20 that are not previously used in Mark's Gospel, which Metzger and others use as proof that Mark didn't write it. Yet, if you take the previous 12 verses in Mark (15:44-16:8) and apply the same criteria you find that this section also uses 17 new words that did not previously appear in Mark!7
The arguments against authenticity seem to be rooted in the writings of Eusebius,4 who, ironically, seems to have favored the inclusion of these verses.5 Regardless, both external evidence and internal structural evidence weigh in favor of the authenticity of Mark 16:9-20. Some have argued that internal stylistic evidence points to the contrary, but upon closer examination, these claims seem to be weak at best. Stylistic clues should be considered, but only with the utmost caution, given the nature and track-record of this method of analysis.
Thus, the evidence -- both internal and external -- seems to indicate that Mark 16:9-20 was, in fact, originally part of Mark's Gospel.
1: http://www.soniclight.com/constable/notes/pdf/mark.pdf, pp. 206-207
2: http://www.bible-researcher.com/endmark.html citing A Commentary on the Holy Bible, edited by J.R. Dummelow (New York: MacMillan, 1927), pages 732-33
3: http://www.bible-researcher.com/endmark.html citing Bruce Metzger, A Textual Commentary on the Greek New Testament (Stuttgart, 1971), pages 122-126
4: http://www.bible-researcher.com/endmark.html citing F.H.A. Scrivener, A Plain Introduction to the Criticism of the New Testament, fourth ed. (London: George Bell and Sons, 1894), volume 2, pp. 337-344
5: https://www.apologeticspress.org/apcontent.aspx?category=13&article=704 citing Aland, Kurt and Barbara Aland (1987), The Text of the New Testament (Grand Rapids, MI: Eerdmans), page 287
7: https://www.apologeticspress.org/apcontent.aspx?category=13&article=704 citing Broadus, John A. (1869), “Exegetical Studies,” The Baptist Quarterly, 3:355-362, July.
If you have not read 'The last twelve verses of Mark' from Dean John Burgon, I'm sad to say that you have not fully researched this subject. Please read it, it will honestly vindicate these verses as a true part of the Holy Scriptures as they truly are. This book has not been fully answered by the critics since it publication over a hundred years ago, simply because it takes the right stand for the Word of God. It's too long to put into this little answer box. The book.
Another link to the book in PDF format.
The Last Twelve Verses of the Gospel According to S. Mark Vindicated Against Recent Critical Objectors and Established - John William Burgon
Besides the Gothic and Egyptian versions in the ivth century; besides Ambrose, Cyril of Alexandria, Jerome, and Augustine in the vth, to say nothing of Codices A and C;—the Lectionary of the Church universal, probably from the second century of our æra, is found to bestow its solemn and emphatic sanction on every one of these Twelve Verses. They are met with in every MS. of the Gospels existence, uncial and cursive,—except two; they are found in every Version; and are contained besides in every known Lectionary, where they are appointed to be read at Easter and on Ascension Day.
The only two missing these verses are the corrupt א and B.
What there is in this to countenance the notion that in the opinion of Eusebius “the Gospel according to S. Mark originally terminated at the 8th verse of the last chapter,”—I profess myself unable to discover. I draw from his words the precisely opposite inference. It is not even clear to me that the Verses in dispute were absent from the copy which Eusebius habitually employed. He certainly quotes one of those verses once and again [see note below]. On the other hand, the express statement of Victor of Antioch [A. D. 450?] that he knew of the mutilation, but had ascertained by Critical research the genuineness of this Section of Scripture, and had adopted the Text of the authentic “Palestinian” Copy,—is more than enough to outweigh the faint presumption created (as some might think) by the words of Eusebius, that his own copy was without it. And yet, as already stated, there is nothing whatever to shew that Eusebius himself deliberately rejected the last Twelve Verses of S. Mark’s Gospel. Still less does that Father anywhere say, or even hint, that in his judgment the original Text of S. Mark was without them. If he may be judged by his words, he accepted them as genuine: for (what is at least certain) he argues upon their contents at great length, and apparently without misgiving.
[Note] The reader is referred to Mai’s Nov. PP. Bibl. vol. iv. p. 262, line 12: p. 264 line 28: p. 301, line 3-4,, and 6-8.
The CNTTS database shows that the following manuscripts contain
verse 9 A C D05 E07 G011 K017 L019 M021 S U Wsupp Y D Q P Y W 1 2 13 28 33 35 69 118 124 157 346 565 579 700 788 1005 1071 1424 1582 2358 2372
and then jumping to verse 20 A C D05sup E07 G011 H013 K017 L019 M021 S U Wsupp Y D Q Psup Y W 1 2 13 28 33 35 69 118 124 157 346 565 579 700 788 1005 1071 1424 1582 2358
So at least above all of these the testimony of Codices Sinaiticus and Vaticanus are taken to be the true and therefore these God-inspired verses are left out.
After all that has gone before, our two oldest Codices (Cod. B and Cod. א) which alone witness to the truth of Eusebius’ testimony as to the state of certain copies of the Gospels in his own day, need not detain us long. They are thought to be as old as the ivth century: they are certainly without the concluding section of S. Mark’s Gospel. But it may not be forgotten that both Codices alike are disfigured throughout by errors, interpolations and omissions without number; that their testimony is continually divergent; and that it often happens that where they both agree they are both demonstrably in error. Moreover, it is a highly significant circumstance that the Vatican Codex (B), which is the more ancient of the two, exhibits a vacant column at the end of S. Mark’s Gospel,—the only vacant column in the whole codex: whereby it is shewn that the Copyist was aware of the existence of the Twelve concluding Verses of S. Mark’s Gospel, even though he left them out: while the original Scribe of the Codex Sinaiticus (א) is declared by Tischendorf to have actually omitted the concluding verse of S. John’s Gospel,—in which unenviable peculiarity it stands alone among MSS.