Why should questions like this matter to ordinary believers? Are they not best left to experts?
It seems to me that they should matter to ordinary believers, first of all, because a passage like this contains precious truth about our Lord Jesus Christ. If we love him, should we not be concerned to treasure up everything that God has revealed about him? If we receive a letter from a beloved wife or child or friend, is it of no consequence to us to hear someone insinuate that the letter has been altered, and that what it says about our beloved is not true? How much more, then, when what is at stake is the precious Word of God, and the words and deeds of our beloved Saviour, to whom we owe everything, and on whom all our hopes for eternity depend?
Then also, such questions concern the honour and truthfulness and reliability of our Saviour. Has he not declared, 'Heaven and earth shall pass away, but my words shall not pass away' (Matthew 24:35)? But if the genuine ending of Mark is lost, some of his words have passed away, and those we have at the end of Mark may not be his words at all, but the invention of a scribe.
A great deal might be said about the passage in Mark, but I would only say, if you are in doubt and difficulty about it, read John William Burgon's great book, The Last Twelve Verses of Mark, first published in 1871, and available in print or on the Web. I have a copy I purchased in Glasgow in 1968. It is falling to pieces, but I am trying to preserve it. Burgon disposes of most of the objections to the passage and gives very convincing reasons why it was omitted at an early date. Trust in Christ, not the critics!
Firmer than earth thy gospel stands,
My Lord, my Hope, my Trust;
If I am found in Jesus’ hands,
My soul can ne’er be lost.
His honour is engaged to save
The meanest of his sheep;
All that his heavenly Father gave
His hands securely keep.
Nor death, nor hell shall e’er remove
His favourites from his breast:
In the dear bosom of his love
They must for ever rest.