Albert Barnes Commentary Mark 16:18

"they shall take up serpents, and if they drink any deadly thing, it shall in no wise hurt them; they shall lay hands on the sick, and they shall recover." — Mark 16:18 (ASV)

They shall take up serpents. When it is necessary for the sake of establishing religion, they shall handle poisonous reptiles without injury, thus showing that God was with him, to keep them from harm. This was literally fulfilled when Paul shook the viper from his hand .

Any deadly thing. Any poison usually causing death.

Shall not hurt them. There is a similar promise in Isaiah 43:2.

They shall lay hands on the sick, etc. See instances of this in the Acts of the Apostles (Acts 3:6–7; Acts 5:15 and following).