This question isn't going to work. They are likely to say, for example, that the prophecies are so vague as to be interpreted in different ways to make them fit or that they were written after the event. Also, if an atheist believes in the supernatural, predicting the future is compatible with the non-existence of God. I'm not sure what i'd ask instead. I quite possibly wouldn't bother because this kind of question, at best, appeals to logic without engaging the emotions or reputation, which isn't the best strategy.