The question arises because in the most ancient manuscript evidence, Mark's Gospel ends at chapter 16:8. The signs are that extra verses were added later. Most modern bibles tell the reader so in the footnotes, so that the reader knows that Mark 16:9-20 was probably not originally part of Mark's Gospel. (There are other alternative endings too, but I'm trying to keep this simple.)
The messenger’s promise to be conveyed to Peter and the others about what to expect in Galilee is, “There you will see him, just as he told you.” So the promised appearance in Galilee links to something Jesus said earlier to them. (Mark’s Gospel is actually laced with the promise and expectation of Jesus’ resurrection. See Mark 8:31, 9:31, 10:34 and especially Mark 14:28 about the promise of the risen Jesus going to Galilee.)
In verse 8, the women run from the tomb in fear, and, we are told, “They said nothing to anyone,” except of course that they obviously did tell, otherwise their great moment at the tomb would not be found here. So we are left anticipating the promised appearances of the risen Jesus in Galilee. In light of that story and promise, you can’t close the book there and say that Mark doesn’t believe that. Mark is clearly a believer and clearly believes that the appearances of the risen Jesus in Galilee are bona fide. But we don’t get to hear more, because that is where the text of Mark’s Gospel breaks off.
- when: Sunday morning
- where: the empty tomb
- who is there: the women and a messenger
- what: Jesus has risen
- why: it is as Jesus foretold
- what Jesus is doing now: “He is going ahead of you into Galilee.”
- what will happen: Jesus will appear
- to whom he will appear: Peter and the disciples
- where he will appear: Galilee
- when he will appear: after they arrive in Galilee.
As NT Wright observes, one place where we find Mark's promised appearances of Jesus in Galilee is in the ending of Matthew's Gospel. And since Matthew re-uses 95% of the material provided to him by Mark, then this makes it all the more likely that Matthew's description of the Galilean appearances are at least in part derived from Mark. In fact, it reads well if you tag onto the end of Mark some verses from Matthew 28. Taking a bare minimum, in fact, you would get this, where the material flows from Mark 16:8 seamlessly into the words from Matthew:
I've minimised material taken from Matthew, just to show how it flows. I could have used more. It gives an impression of what the original ending of Mark could very possibly have looked like. I am not saying it was so, just that it could have been so, given that it flows naturally, and that this delivers what Mark promises, and that Matthew reuses 95% of Mark's material, so the wording in the latter paragraph above, which is found in Matthew, could have been derived from Mark's original ending.