The verses in question are Mark 16:9-20, not 12-16. And there are two issues:
1) Are the verses part of Mark's Gospel? (manuscript evidence, style, vocabulary, etc. have to be examined).
2. If they are not part of Mark's Gospel originally, are they still part of the canon of Scripture.
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The issue is really about what manuscripts have - and it is not a simple case of claiming that only two (Sinaiticus and Vaticanus) are missing these verses. Here is a summary of the manuscript evidence:
[note: manuscript refers to a hand written copy of the Scriptures essentially before 1500 (printing press in 1453).
1.* * * * No ending
Manuscripts: Sinaiticus, Vaticanus, 304 syriac(a), Coptic (sahidic), Armenian (~100 mss), Georgian (two traditions),Clement, Origen, Eusebius. And Eusebius and Jerome both note that most Greek manuscripts did not include an ending - even the numbering scheme used in the Eusebian writings make no numeric provision for anything beyond 16:8.
2.* * * * Shorter ending of two verses (sometimes given in footnotes),
"But they reported briefly to Peter and those with him all that they had been told. And after this Jesus himself sent out by means of them, from east to west, the sacred and imperishable proclamation of eternal salvation."
Manuscripts: L, Psi, 099, 0112, it(k), syriac(Harclean margin), 274(margin), 579, lectionary (1602), coptic (several Sahidic mss, several Coptic mss), several ethiopic mss.
Note: all of these except it(k) also then include the longer ending below.
3.* * * * Longer ending (vs. 9-20)
Manuscripts: A, C, D, K, Chi, Delta, Theta, Pi, f(13), 28, 33, 565, 700, 892, 1009, 1010, 1071, 1079, 1195, 1230, 1242, 1253, 1344, 1365, 1546, 1646, 2148, 2174, many Byzantine mss, lectionaries (60, 69, 70, 185, 1761), it (Aureus, Colbertinus, Bezae Cantabrigiensis, Corbeiensis II, Rehdigeranus, Sangellensis [5th cent and 7th cent], q[13] Monacensis), vulgate (Clementine and Wordsworth-White), syriac (Curetonian, Peshitta, Harclean, Palestinian), coptic (Sahidic, Boharic, Fayyumic), gothic, armeinian (several mss), ethiopic (several mss), georgian (Blake), Diatesseron (a, i, n), Irenaeus (Greek, Latin), Tertullian, Aphraates, Apostolic Constitutions, Didymus
4.* * * * Longer ending (vs. 9-20) plus addition in vs. 14
"And 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.' thus they spoke to Christ. And Christ answered them, 'The term of years of Satans 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 inheirt the spiritual and incorruptible glory of righteousness which is in heaven.' "
Manuscripts: W, (and attested to its presence by Jerome)
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One thing that is missed in the proposal that items would be more likely to be deleted is then to explain how the other textuals differences arose. Namely, why take something that was "smooth" syntactically or grammatically, or more complete, and then chop it up so that it becomes difficult to understand?
So, how to address these issues? I will only comment briefly now by adding what two commentators have provided as an assessment of this evidence. The style is definitely not Markan. Henry Barclay Swete. Commentary on Mark: The Greek Text with Introduction, Notes, and Indexes, 1913] notes that Mark 16:9-20 showed evidence that the longer ending is part of a self-contained unit that was assimilated sometime in the late 2nd century to Mark's Gospel. Also, interesting because Swete asserts that the style of the longer ending (vs. 9-20) is Johannine in style rather than Markan.
William L. Lane The Gospel According to Mark: The English Text with Introduction, Exposition, and Note, 1974] provides a terse summary of issues regarding the shorter and longer endings of Mark, claiming that evidence indicates that the shorter ending was probably written prior to the longer one. Vocabulary of the longer ending is marked by 17 non-Markan words (p. 604, fn 10). Lane also notes that "Mark's usual transitions, EUQUS, PALIN are absent from verses 9-20; the use of KAI is rare and no phrase begins with parataxis."
That ending of the gospel at Mark 16:8 is consistent with the theology that Mark presents problems. No reason to suspect that Mark once had additional material, now lost, that may have included resurrection appearances. Mark has no need for them, and it weakens Mark's theme of Gods power to accomplish that which Christ's mission and the Good News of Christ sets out to do, and which is done by God's action and bidding alone. It also weakens the theme of God being an active force in the world, out there, working ahead of us, like Jesus in Galilee, like the Spirit let out of heaven, God on the loose.
One possible explanation (again, only conjecture) has been offered that Mark had taken a break from writing and never went back to it. That is, Mark had been arrested and died before finishing. After 20+ years of study and being all over the board on this issue, I personally have come to this position. But am open to further study and interaction.
Another thought is that ending on a fear is not typical "gospel" stuff. But that too might fit with his own identifying mark (poor pun) in chapter 14 when he runs away (naked) in fear.
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Thus as far as the first question above: the evidence suggests that these verses were not part of Mark's Gospel originally. As for the second question, the evidence is clear that it is canonical. that is, by the time of the final formalization of the canon as we know it today, those verses were considered part of the canon.