These verses appear in certain Bible manuscripts and versions of the fifth and sixth centuries C.E. But they do not appear in the older Greek manuscripts, the Sinaiticus and Vatican MS. 1209 of the fourth century. Dr. B. F. Westcott, an authority on Bible manuscripts, said that “the verses . . . are no part of the original narrative but an appendage.” (An Introduction to the Study of the Gospels, London, 1881, p. 338) Bible translator Jerome, in the fifth century, said that “almost all the Greek codices [are] without this passage.” (The Last Twelve Verses of the Gospel According to S. Mark, London, 1871, J. W. Burgon, p. 53)
Reasoning From the Scriptures. 1985, 1989 ed. p. 159