Is It Authentic?
In most newer translations, the last twelve verses of Mark 16 are set apart from the rest of the text. This is to indicate that their authenticity has been questioned. However, the evidence overwhelmingly supports the inclusion of the latter verses. Before looking at the evidence, let us note that there is nothing taught in the latter verses of Mark 16 that cannot be established by other parts of the New Testament. The resurrection of Christ on the first day of the week, the necessity of belief and baptism, the working of miracles by the apostles, etc. are all taught elsewhere.
The latter verses of Mark 16 do not appear in two of the oldest manuscripts (Sinaiticus and Vaticanus). However, they do appear in many other early manuscripts. They appear in Codices Alexandrian, Washington, Ephraim, etc. To suggest that there is no manuscript support for the inclusion of the latter verses is disingenuous. Furthermore, several early writers either quote from or allude to the latter verses in their writings (Justin Martyr, Irenaeus, etc.), which predate even the earliest manuscripts!
It is absurd to think that Mark ended his gospel with the disciples “afraid” (v. 8). Where is the triumph in that? The latter verses provide a natural and triumphant end to this masterful account. Mark 16:16 and the other verses in question are authentic.