It is virtually certain that Mark 16:9-20 is a later addition and not the original ending of Mark. The evidence is numerous and complex.
Since none of the original autograph copies of NT documents survive, the Greek text of the NT is constructed from later copies of manuscripts dating from AD 135 to AD 1200. There are over 5000 of these, which range from scraps the size of a postage stamp, to complete manuscripts of the Bible. In general, these copies show remarkable agreement! The notorious exception to this happy rule, is the ending of Mark, which presents the gravest textual problem in the NT.
The two oldest and most important manuscripts, Codex Vatican's (B) and codex Sinaiticus (Alef) omit 16:9-20 as do several early manuscripts or versions, including the Old Latin and the Synaptic Syriac manuscript, about 100 Armenian manuscripts and the two oldest Georgian manuscripts. Neither Clement of Alexandria nor Origen shows any awareness of the longer ending, and Eusebius and Jerome attest that vv 9-20 were absent from the majority of Greek copies of Mark known to them. An ingenious system of cross-referencing parallel passages in the Gospels was devised by Ammonius in the 2nd century and adopted by Eusebius in the fourth century, which do not contain Mark 16:9-20.
The fact that some manuscripts include the longer ending, means it was added quite early, and then incorporated into subsequent copies of the gospel. However, even when this ending is included, it is often noted that the ending is regarded as a spurious addition to the Gospel. External evident (manuscript witnesses) thus argues strongly against the originality of the Longer Ending.
Literary criticism or internal evidence also argues against the Longer Ending! In verse 16:8, the subject is the frightened and fleeing women, verse 9 begins by presupposing the resurrected Jesus,who appears to Mary Magdalene. She is presented as a "newcomer" ("out of whom [Jesus] had driven 7 demons") although Mark had mentioned her three times before (15:20, 47; 16:1) In vv 9-20 Jesus is for the first time in Mark referred to as the "Lord Jesus" (19) or the "Lord" rather than Mark's custom of calling Jesus by his given name in the rest of Mark. Such reverential nomenclature likely derives from later Christian worship! (In other words, a high Christology not present in Mark, and thus an anachronism.)
Particularly noticeable is the namer of new words that appear nowhere else in Mark. There are 52 new words, found no where else in Mark, in only 11 verses. Plus, there are several unique word forms and syntactical constructions. Several of Mark's stylistic features are likewise absent from the longer ending, notably, the absence of the kai "και" in Mark's sentence structure and the absence of historical present tense of verbs and the absence of euthys.
The longer ending also contains peculiar themes to itself,, some of which contradict Markan themes. The repeated chastisement of the disciples for their "disbelief", of the gospel proclamation, is unique to the longer ending, and the prominence given to charismatic signs in vv 17-18 in stark contrast to the reserve of Jesus in Mark with regard to signs and sensation.
Plus these themes of drinking poison and handling snakes are not found anywhere else in Scripture, except perhaps for the "involuntary" bite of the viper with Paul in Acts 28:3-6. The expectation of both having some supporting manuscript evidence from elsewhere in the NT, and that these two activities are supposed to be regular "signs" even though not talked about elsewhere. As I often say, you cannot make a doctrine out of one appearance of something, especially when it is as bizarre as drinking poison and found no where else, really points to the fact that the longer ending is not valid.
In many respects, vv 9-20 have something of a "secondhand" flavour, and it looks like a pastiche of elements drawn from the other gospels and Acts.
The style of the Longer Ending as a whole reads very differently from Mark's lively and expansive narrative, and contains a noticeable concentration of words not found elsewhere in Mark. In particular, v 20 reads more like a pious committee summaries of the post-Easter task and experience of the church, than like the way Mark writes his gospel.
For these reasons, it is the almost unanimous conclusion of scholars that the Longer Ending represent well-meaning attempts, probably some time in the 2nd century, to fit in the perceived gap left by the "unfinished" ending at 16:8, by drawing eclectically on what had by then become the familiar traditions of the post-apostolic church. Thus, the Longer Ending of Mark is an incongruous addition to the gospel.
The Gospel According to Mark: The Pillar New Testament Commentary, James, R Edwards, William B Eerdemans Publishing, Grand Rapids, Michigan, 2002.
The Gospel of Mark: The New International Greek Testament Commentary, R. T France, William B Eerdmans Publishing, Grand Rapids, Michigan, 2002.
