According to Luke, the people thought Paul would get sick or die after a snake bit him, but when nothing bad happened to him, they were amazed and started to believe he was a god. This story shows how God's protection can change people's minds.
Acts 28:6: Howbeit they looked when he should have swollen, or fallen down dead suddenly: but after they had looked a great while, and saw no harm come to him, they changed their minds, and said that he was a god.
While Acts 28:6 describes an instance where Paul is unharmed by a snake bite, Mark 16:18 suggests that believers, in general, will not be harmed by serpents, but it is not addressing a miraculous expectation.
Mark 16:18: They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.