There must be a mistake somewhere. In fact, that is what nearly all modern Bible scholars have concluded, namely, that these words—and not only these words but all of what appears as Mark 16:9-20—were not written by Mark but were added by a later hand.
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) The New Catholic Encyclopedia (1967) says: “Its vocabulary and style differ so radically from the rest of the Gospel that it hardly seems possible Mark himself composed it [that is, verses 9-20].” (Vol. IX, p. 240) There is no record that early Christians either drank poison or handled serpents to prove they were believers.