Mark's Alternate Ending--Why is it in my Bible?
The Longer Ending in Mark…
Main Point: We need not worry or stress over the last verses in Mark, though not scriptural, such verses are neither erroneous or dangerous.
How do we know this verses are not original to Mark?
Neither Clement of Alexandria nor Origen shows any awareness of the existence of the longer ending, and Eusebius and Jerome attest that vv. 9–20 were absent from the majority of Greek copies of Mark known to them. An ingenious system of cross-referencing parallel passages in the Gospels that was devised by Ammonius in the second century and adopted by Eusebius in the fourth century (hence the name Eusebian Canons) does not include Mark 16:9–20.
Why add these verses?
Although the longer ending is clearly secondary, it is nevertheless very old. The earliest witnesses to the longer ending come from the Epistula Apostolorum 9–10 (c. 145), perhaps Justin Martyr (Apol. 1.45; c. 155), Tatian’s Diatessaron (c. 170), and Irenaeus (Adv. Haer. 3.9–12; c. 180). This means that the longer ending “must be dated to the first decades of the second century.”