There are questions about the how Mark’s gospel concludes. By that, I mean there is a question about whether the material in 16:9-20 are part of the original text or if it was added at some later date.
The NIV has this note before v. 9-20:
The earliest manuscripts and some other ancient witnesses do not have these verses.
It’s a somewhat involved discussion, but I think the note in the NIV is a fair, albeit extremely brief summation. There are legitimate questions about whether these verses were part of Mark’s original gospel. I think Mark’s original ending (whatever was originally after v. 8) got lost somewhere along the way (but not before accomplishing God’s purpose). The material in v. 9-20 was added at a later date and might reflect quite closely Mark’s original ending—we just don’t know.
Here’s what we do know: there’s nothing in the longer ending we have any doubt about in terms of its truthfulness.
In fact, everything in v. 9-20 can be found elsewhere in the Scripture. The appearance to Mary Magdalene (John 20:11ff), the two disciples on their way to Emmaus (Luke 24:13ff), to the apostles (Luke 24:36ff), Jesus’ commissioning the apostles (Matthew 28:16ff), miraculous signs (the book of Acts), Jesus’ ascension (Luke 24:50ff), and confirmation of the apostles’ message through signs (Hebrews 2:1-4). So, there’s no doubt about the truthfulness of anything said in v. 9-16. Therefore, whether you accept the long ending or the short ending—there’s no question about the veracity of either.
To recap, we’ve said there are legitimate questions about the authenticity of Mark 16:9-20 (i.e., whether it was part of the original text), but there are no questions about the truthfulness of the material in those verses. So, either ended is possible.
But neither ending is really the point.
The real point of Mark 16 (according to either ending), is that “HE HAS RISEN!” (v. 6). There’s absolutely no doubt about that in Mark’s account and it functions as the dominant truth of the section. While textual criticism is important and we must always treat all questions it raises with due diligence, we don’t want to be guilty of straining out gnats and swallowing camels. No truth of Scripture is under dispute and one of the most central truths is revealed—“HE HAS RISEN!” We need to treat this as the glorious treasure it is. We don’t want to get all caught up worrying about whether it has an 8-verse setting or a 20-verse setting. It’s simply the most beautiful, amazing diamond we’ve ever seen and we need to treat it that way.