Four theories have been put forth to explain this, each of which has been defended by serious, Bible-believing scholars:
- Verses 9-20 are original to Mark, but are simply missing from Sinaiticus and Vaticanus. In support of this theory is the fact that Justin Martyr (d. 145) and Tatian (in his Diatesseron from about 170) as well as Iraneus and Hippolytus (Church Fathers from the 2nd and 3rd centuries) quote from these verses. In addition, almost all of the other manuscripts from the 5th century and later include verse 9-20. It is thus quite possible that these later manuscripts (from the 4th century) were copying from texts whose last page was missing.
- Mark finished his Gospel and it went beyond verse 8, but the original ending was lost before it was copied, so verses 9-20 were added later to finish it. There is almost no way of proving this, but it is a logical possibility.
- Mark didn't finish his Gospel for some reason (such as sudden death), and a later writer added verse 9-20. Again, this is possible, but there is no way of proving it.
- Mark purposely ended at verse 8, but a later editor added verses 9-20. This seems difficult, though possible, for the simple reason that verse 8 does seem like an odd place for the story to end.
My conclusion based on the evidence we have is that either Mark ended his Gospel at verse 8 or that his original ending (which was more similar to Luke or Matthew) was lost at an early stage of transmission and that verses 9-20 were added by a later editor.
This conclusion may raise an additional question in some people's minds, (i.e., "But if verses 9-20 aren't original, are they Bible?") but that is a subject which merits its own post. If you've made it this far, you can surely hang on for one more!