Good points listed already. Here's a couple of points I've used:
- (Let's say I still think the bible is relevant.) The Bible contains many cautionary tales of the errors that MEN have made. Here the disciples jumped to a wrong conclusion. In 2 Kings 22:20, a "true prophet" told Josiah that he would be gathered to his "own graveyard in peace". But he didn't die in peace, he died in a war with Egypt. Another example that comes quickly to mind is the lying prophet from Bethel. So are we to conclude when someone claims to predict the future? They will be wrong. And from some of these examples, following these people can kill us.
- (More accurately, since I don't think the bible is relevant.) Oh, so you're saying that Jesus inspired false prophecy among his disciples? Is this your plan to build my confidence in the bible by pointing out how it is chock full of errors? And that I should have confidence in the false prophecies of WT because the bible also has false prophecies?