This is as big (bigger) problem for Protestants than it is for Roman Catholics - for very different theological reasons. In the Protestant church it can simply be omitted, but it isn't. It's the same basic practical issue, questioning this passage upsets Christians. In Protestant churches it complicates the common understanding of the inerrancy.
It's useful to clear the background to the question. My understanding of the council of Trent (I'm not RC).
- It was a very significant event in response to the Reformation.
- The interpretation subject to huge debate much later, Vatican I sort of thing.
- It declared the Latin Vulgate as adequate for doctrinal proofs rather than the original Hebrew and Greek manuscripts
Thus, Mark 16:9-20 is therefore part of the Latin Vulgate, as it is in Protestant bibles. Whatever the Vulgate was at the Council of Trent, thats the Roman Catholic bible, because the issues post-council of Trent concern Tradition et Scripture, not what is the relevance of Mark 16:9-20.
The timing of the Council is relevant as to whether the issue was widely known. Moreover, how much authority at the time would have been placed on a single book? The real underlying issue surfaced when supporting codices were discovered much later notably in 1844 and then 1892.
I would politely suggest there is very little evidence for Mark 16:9-20 being part of the original Gospel of Mark. The arguments are well known:
- the discovery of the codex Sinaiticus (1844) and the Codex Sinaiticus Syriacus (1892) confirmed codex Vaticanus. Thus, all the earliest codices are in agreement.
- I believe the Codex Sinaiticus Syriacus contains the words "The End" after Mark 16:8, which seems pretty strong evidence.
- Obviously the style of writing is completely different to the Gospel of Mark.
- There are difficulties with the contents of the passage:
- Drinking deadly poisons and snake bites: I would suggest of all the Christian blessings immunity from poisons and snake bites does not appear high on the list of priorities. The Council of Trent concerned salvation, rather than clinical toxicity. Moreover, I would advise any individual suffering toxicity to seek immediate medical attention rather than proclaim Scriptural authority over a disputed passage.
- Jesus rebukes his disciples. This is not the Jesus of the bible. The biblical Jesus has incredible patience, and never collectively (stress intentional) rebukes his disciples. It also seems to contradict Jesus' restoration of the leadership of Peter at the end of the Gospel of John. If you were going to rebuke someone, Peter's denial of Jesus seems pretty good grounds for doing so.
There are additional arguments of context.
- The Gospel of Mark starts with a bang, a vagrant in the middle of a desert dressed in camel hair, very little historical context;
- it ends with a bang at Mark 16:8.
- The repetition of predicting the resurrection is in the build up to this dramatic conclusion.
- Arguments such as "the original ending went missing" purported on grammatical criteria have not stood up to subsequent analysis on bigger sample sizes of ancient Greek correspondence.
Thus what I am leading up to is simply that @RevelationLad's response is the most consistent answer from a Roman Catholic perspective. Here the authority of the Council of Trent is clear. Ultimately, I would (obviously) argue its about individual belief.
By tradition the Gospel of Mark was written under the authority of Peter. I think the ending is very much in character of Peter. He never really appeared to consider the long term ramifications of short term decisions. If you want to get people into your church, ending on a cliff-hanger is exactly how to do it. Thus, I think it points to the Gospel of Mark being written and distributed when the witnesses to the resurrection were still alive. These are just thoughts however not an argument for the Gospel of Mark ending on 16:8 and the discussion of dating when the Gospel of Mark was written is a separate question.