Mark 16:9–20 are the last twelve verses of the Gospel of Mark. These verses aren't included in two of the oldest and most reliable fourth-century Greek biblical manuscripts; Codex Sinaiticus and Codex Vaticanus. It’s almost unimaginable that the copyists who made them weren't aware of Mark 16:9–20, but they left it out of their Bibles. Modern Bible versions often include it in brackets or footnotes, such as ESV, NLT, NIV, and NASB. Jesus describes there the signs that would follow those who believe him (Mark 16:17–18). It’s important to know that including a note here isn't a recent development in church history. Christians have known for centuries that Mark 16:9–20 may not have originally been part of the Gospel of Mark.
When we look at the manuscripts of the Gospel of Mark that survive today, more than 99 percent contain Mark 16:9–20. This includes not only over 1,600 Greek manuscripts, but also most manuscripts of early translations of the Gospel of Mark.
Further, around 180 AD, Irenaeus unambiguously quoted Mark 16:19 as Scripture in Against Heresies. Justin Martyr and Tatian also knew the verses earlier in the second century. Undeniably, many early Christians considered Mark 16:9–20 to be part of the Gospel of Mark. These persons must have quoted from an earlier manuscript that we no longer have available, older than Codex Sinaiticus and
Codex Vaticanus. This is strong evidence that Mark 16:9–20 was part of the original text, inspired by God. See this explained at time 2:24–4:41 in
Mark 16:9-20 – What Does Jesus Want Me to Do?
My personal opinion is therefore that Mark 16:9–20 was part of the original text, but later wasn't included in Codex Sinaiticus and Codex Vaticanus.
For detailed information, see Was Mark 16:9–20 Originally Part of Mark’s Gospel?
See also Critical verses in the gospels at the end of The Four Gospels.
