Although the vast majority of later Greek manuscripts contain Mark 16:9-20, the Gospel of Mark ends at verse 8 in two of the oldest and most respected manuscripts, the Codex Sinaiticus and Codex Vaticanus. As the oldest manuscripts are known to be the most accurate because there were fewer generations of copies from the original autographs (i.e., they are much closer in time to the originals), and the oldest manuscripts do not contain vv. 9-20, we can conclude that these verses were added later by scribes. The King James Version of the Bible, as well as the New King James, contains vv. 9-20 because the King James used medieval manuscripts as the basis of its translation. Since 1611, however, older and more accurate manuscripts have been discovered and they affirm that vv. 9-20 were not in the original Gospel of Mark.
In addition, the fourth-century church fathers Eusebius and Jerome noted that almost all Greek manuscripts available to them lacked vv. 9–20, although they doubtless knew those other endings existed. In the second century, Justin Martyr and Tatian knew about other endings. Irenaeus, also, in A.D. 150 to 200, must have known about this long ending because he quotes verse 19 from it. So, the early church fathers knew of the added verses, but even by the fourth century, Eusebius said the Greek manuscripts did not include these endings in the originals.
The internal evidence from this passage also casts doubt on Mark as the author. For one thing, the transition between verses 8 and 9 is abrupt and awkward. The Greek word translated “now” that begins v. 9 should link it to what follows, as the use of the word “now” does in the other synoptic Gospels. However, what follows doesn’t continue the story of the women referred to in v. 8, describing instead Jesus’ appearing to Mary Magdalene. There’s no transition there, but rather an abrupt and bizarre change, lacking the continuity typical of Mark’s narrative. The author should be continuing the story of the women based on the word “now,” not jumping to the appearance to Mary Magdalene. Further, for Mark to introduce Mary Magdalene here as though for the very first time (v. 9) is odd because she had already been introduced in Mark’s narrative (Mark 15:40, 47, 16:1), another evidence that this section was not written by Mark.
Furthermore, the vocabulary is not consistent with Mark’s Gospel. These last verses don’t read like Mark’s. There are eighteen words here that are never used anywhere by Mark, and the structure is very different from the familiar structure of his writing. The title “Lord Jesus,” used in verse 19, is never used anywhere else by Mark. Also, the reference to signs in vv. 17-18 doesn’t appear in any of the four Gospels. In no account, post-resurrection of Jesus, is there any discussion of signs like picking up serpents, speaking with tongues, casting out demons, drinking poison, or laying hands on the sick. So, both internally and externally, this is foreign to Mark.
While the added ending offers no new information, nor does it contradict previously revealed events and/or doctrine, both the external and internal evidence make it quite certain that Mark did not write it. In reality, ending his Gospel in verse 8 with the description of the amazement of the women at the tomb is entirely consistent with the rest of the narrative. Amazement at the Lord Jesus seems to be a theme with Mark. “They were amazed at his teaching” (Mark 1:22); “They were all amazed, so that they debated among themselves” (Mark 1:27); “He healed the paralytic, and they were all amazed and were glorifying God saying, ‘We’ve never seen anything like this’” (Mark 2:12). Astonishment at the work of Jesus is revealed throughout Mark’s narrative (Mark 4:41; 5:15, 33, 42; 6:51; 9:6, 15, 32; 10:24, 32; 11:18; 12:17; 16:5). Some, or even one, of the early scribes, however, apparently missed the thematic evidence and felt the need to add a more conventional ending.