“…and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.”  Mark 16:18

When you got born again, you received the life of God. This is why God sent Jesus; why Jesus came, to give you his life:
“For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.”  John 3:16
That life that you received supplanted the human life and as such all of your physical body- internal and external organs- are vitalized by this new life.
If the old life was okay, this new life far surpasses ‘okay’.
If you were injured and you had the old life, depending on the size, If it was a mild bruise, it’ll get healed in a few days, but if your ear was cut off intentionally or by accident, it’s gone! We believed that until a man with the new life you have received proved what this new life can do:
“And one of them smote the servant of the high priest, and cut off his right ear. And Jesus answered and said, Suffer ye thus far. And he touched his ear, and healed him.” Luke 22:50-51
If he administered healing to another person, then imagine if it happened to him!
With the old life, deadly things cause death, thus the term ‘deadly’, they affect your digestive system in such a way that would prove fatal.
When you get born again, the life vitalizing your organs and systems have made them corruption free, so he said: ‘and if they drink any deadly thing, it shall not hurt them’
No poison can affect your digestive system; it is sustained by a new life!
So Jesus concludes, since you are untouchable, go and help others who can still be affected by deadly things:
‘…they shall lay hands on the sick, and they shall recover.’
Believe him!