What the manuscripts show. Codex Sinaiticus and Codex Vaticanus, both of the fourth century, end at 16:8: “for they were afraid.” In Vaticanus a blank column follows. Some early translations also end there: the Sinaitic Syriac and some Sahidic, Armenian and Georgian manuscripts. In the early fourth century Eusebius of Caesarea wrote that the accurate copies, indeed almost all copies he knew, ended at that point. These facts are secure (Very Strong).
The other endings. Most Greek manuscripts, including Alexandrinus, Ephraemi, Bezae and Washingtonianus, together with the Byzantine tradition, most Latin and Syriac witnesses and the Bohairic Coptic, have verses 9–20. A few manuscripts have a short ending of one or two sentences, usually followed by the longer one; one Old Latin manuscript has only the short ending. Codex Washingtonianus adds a further saying after verse 14. Several manuscripts carry notes saying that older copies lacked the verses.
How old is the longer ending? Older than any of these manuscripts. St Irenaeus, writing about 180, quotes verse 19 and says it comes “towards the conclusion of his Gospel” (Against Heresies 3.10.5; 3.10.6 in some editions). Tatian appears to have used it in his harmony of the Gospels. So the longer ending was being read as part of Mark by the later second century (Strong).
Who wrote it? Most textual scholars think verses 9–20 were not written by Mark himself, chiefly because they are absent from the earliest witnesses and from the copies Eusebius trusted (Strong, majority view). A minority defend Markan authorship. Whether Mark meant to end at 16:8, or his original ending was lost, is debated (Debated).
What the Orthodox Church reads. The Patriarchal Text prints the verses as part of the Gospel, and the Church reads them as the third of the eleven Resurrection Gospels at Sunday Matins. For the Orthodox, the authority of these verses rests on the Church’s reception of them as Scripture, not on a modern judgment about authorship.
How translations handle it. The KJV, NKJV and RSV Catholic Edition print the verses in the text; the NKJV adds a note on the manuscripts. The ESV and NRSVUE enclose them in double brackets; the NIV prints them with a note that the earliest manuscripts lack them.
What it does and does not change. Every event in verses 9–20 is attested elsewhere in the New Testament: the appearance to Mary Magdalene (John 20), to two disciples on the road (Luke 24), the great commission (Matthew 28) and the Ascension (Luke 24; Acts 1). Even Paul’s survival of a viper’s bite (Acts 28) parallels verse 18. The resurrection does not depend on this passage.