The disciples did not believe the testimony at first, but then they believed. “When they heard that Jesus was alive and that she had seen him, they did not believe it.” Mark 16:11 NIV https://bible.com/bible/111/mrk.16.11.NIV “But they did not believe the women, because their words seemed to them like nonsense.” Luke 24:11 NIV https://bible.com/bible/111/luk.24.11.NIV Luke 24:11 — The disciples thought the women’s report was nonsense. So Mark 16:16 is about responding to the gospel, not specifically about the disciples’s unbelief after the resurrection. Mark 16:16 “Whoever believes and is baptized will be saved, but whoever does not believe will be condemned.” Mark 16:16 NIV https://bible.com/bible/111/mrk.16.16.NIV