Theophylact of Ohrid
Catena Aurea · Markusevangelium 16
Afterward he appeared unto the eleven as they sat at meat, and upbraided them with their unbelief and hardness of heart, because they believed not them which had seen him after he was risen. And he said unto them, Go ye into all the world, and preach the Gospel to every creature. He that believeth and is baptized shall be saved; but he that believed not shall be damned. And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; 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.
That is, they shall scatter before them serpents, whether intellectual or sensible, as it is said, Ye shall tread upon serpents and scorpions, which is understood spiritually. But it may also mean sensible serpents, as when Paul received no hurt from the viper. There follows: And if they drink any deadly thing, it shall not hurt them. (Luke 10:19) We read of many such cases in history, for many persons have drank poison unhurt, by guarding themselves with the sign of Christ. It goes on: They shall lay hands on the sick, and they shall recorer.