Duke,

Yes, I noticed my error after I posted it, but it was too late. I knew it would be corrected anyway.

Still, I have thought more about this, and we still must tweak it a little for it to be valid.

The initial assumption must be carefully worded so that the premise will not be challenged. This is by no means a final draft, but I think it would go something like this:

"one religion interprets the major points of the other's correctly, and the second religion interprets the major points of its counterpart incorrectly". Now, if you set it like this, it would be logically sound. However, I'm not sure you could convince xians to accept the opening premise, even in generic form before you fill in the names or solve the riddle. In other words, if you went to a xian and said "once upon a time there were 2 groups named xians and jews. One of the religions was true, and the other was false.", surely the xian would agree. Then he would try to demonstrate that his own religion was the true one....but he would agree with the premise. The premise I have suggested is not a slam-dunk to get a xian to accept at the outset.

What do you say?