Can Christians drink deadly poison without being harmed?

In Mark 16:17-18, Jesus says: "And these signs will accompany those who believe: In my name they will drive out demons; they will speak in new tongues; they will pick up snakes with their hands; and when they drink deadly poison, it will not hurt them at all; they will place their hands on sick people, and... show more In Mark 16:17-18, Jesus says:

"And these signs will accompany those who believe: In my name they will drive out demons; they will speak in new tongues; they will pick up snakes with their hands; and when they drink deadly poison, it will not hurt them at all; they will place their hands on sick people, and they will get well."

Thus, according to Jesus, if believers drink deadly poison, it will not hurt them at all.

This would be a good way for Christians to convince doubters to accept Jesus. They could win a lot of converts by demonstrating their immunity to the deadly poison. Why don't they do so?
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