"they will pick up serpents, and if they drink any deadly poison, it will not hurt them; they will lay hands on the sick, and they will recover ."
Of course, the argument follows: "If you Christians truly believe your Bible, you'll never go to the doctor."
Let's look at the WHOLE passage so we can get the CONTEXT, which atheists like to ignore:
"And He said to them, "Go into all the world and preach the gospel to all creation. "He who has believed and has been baptized shall be saved ; but he who has disbelieved shall be condemned. "These signs will accompany those who have believed : in My name they will cast out demons, they will speak with new tongues ; they will pick up serpents, and if they drink any deadly poison, it will not hurt them; they will lay hands on the sick, and they will recover ."
Mark 16: 15-18
It's plain that Jesus was prophesying of Apostolic gifts given to those chosen ones who were seeding the early Church. The prophesy of the snakes came true on one of the Apostle Paul's journeys. He was bitten by a serpent, yet suffered no ill effects.
Many Atheists labor under the delusion that everything in the Bible is a commandment. Where are people commanded here to drink poison, play with vipers, or skip seeing a doctor?
Paul the Apostle knew very well the Apostolic gifts, yet his advice to his student Timothy, concerning what were obviously gastrointestinal issues, was:
"No longer drink water exclusively, but use a little wine for the sake of your stomach and your frequent ailments."
1 Timothy 5:23
Not to mention the Apostle travelled with a Greek doctor named Luke.
There's a REASON we read the Bible in whole passages, versus singling out a verse or two. We believers actually think the Bible is worth reading and understanding.