Here's a challenge for believers. The following comes from Mark Chapter 16 (New American Bible):

17. These signs will accompany those who believe: in my name they will drive out demons, they will speak new languages.

18. They will pick up serpents (with their hands), and if they drink any deadly thing, it will not harm them. They will lay hands on the sick, and they will recover."

I've seen these people claiming to do most of these things but I've never seen a televangilist drinking a deadly poison under controlled conditions, which ought to be a doddle if what they say is true and they really believe it. So I suggest the "Cyanide Challenge" (especially for those disgusting con men selling "The Rapture").

You could offer them a get-out clause: most serious textual critics are convinced that the last twelve verses of Mark (including these) are not original (i.e. they were added by a scribe at some point in the text's history). However, if our Rapture-merchant admits these verses are part of a textual addition, he automatically admits that the Bible is not the inerrant word of God. Either way you've got him.

(Just kidding - these people annoy the hell out of me but I don't want to kill them!)