Bruce Metzger on the longer ending of Mark

“Today we know that the last twelve verses of the Gospel according to Mark (xvi. 9-20) are absent from the oldest Greek, Latin, Syriac, Coptic, and Armenian manuscripts, and that in other manuscripts asterisks or obeli mark the verses as doubtful or spurious. Eusebius and Jerome, well aware of such variation in the witnesses, discussed which form of text was to be preferred. It is noteworthy, however, that neither Father suggested that one form was canonical and the other was not. Furthermore, the perception that the canon was basically closed did not lead to a slavish fixing of the text of the canonical books. Thus, the category of 'canonical' appears to have been broad enough to include all variant readings (as well as variant renderings in early versions) that emerged during the course of the transmission of the New Testament documents while apostolic tradition was still a living entity, with an intermingling of written and oral forms of that tradition. Already in the second century, for example, the so-called long ending of Mark was known to Justin Martyr and to Tatian, who incorporated it into his Diatesseron. There seems to be good reason, therefore, to conclude that, though external and internal evidence is conclusive against the authenticity of the last twelve verses as coming from the same pen as the rest of the Gospel, the passage ought to be accepted as part of the canonical text of Mark.”

Bruce Metzger, The Canon of the New Testament: Its Origin, Development, and Significance (pp.269-270)