An agnostic friend of mine asked me a question and I’m not sure how to respond. His question was “how can we be sure that say… St. Paul wasn’t deceived by demons when he had the supposed vision?”.
Could someone help provide an answer?
Does your friend know he’s not a demon ? Do you think he is ? Can you prove he is not ? Can he ? 
People used to think epileptics were possessed by demons. And there are some who think St. Paul was an epileptic.
We have no way of proving that he was not deceived by demons, or did not have a fit of epilepsy - but neither can we prove that he was not Elvis Presley. There is no need to do so, because there is no basis for thinking that he was. People do not usually waste their time in refuting ideas for which there is no foundation except the idea itself: & one can’t argue from the idea that “Elvis Presley is St.Paul re-incarnated” to the conclusion that he was, because ideas need to be tested in some way if they are to be judged: & having them, is no test of their truth or their rationality or of anything else about them. The same applies to your friend’s idea. Having it, does not validate it.
Presumably no one expects your friend to give proof that would convince even the most sceptical that he is a human being, & not a demon - doesn’t St. Paul deserve the same courtesy of being believed ?
One assumes your friend is not a mathematical genius: few people are, yet such people exist. One also assumes, if he is an agnostic, that he is not a Saint or a mystic: again, few people are, yet such people exist. A modest person realises that although he does not possess certain gifts of mind or body, some people do - & that he cannot argue from his lack of them, to a universal lack of them. It’s not legitimate to reason from one’s own lack of visions or other mystical gifts, to their not having being given to others. If a man born blind insisted that no one else could see either, & that vision was a myth or a wish-fulfilment fantasy, he would be thought less than wise. Yet this is what people do, when they reason from their lack of mystical graces to the non-existence or falsity of those received by the Apostle. Those without the gift of faith, are not equipped to pronounce on the graces granted to those who have received it.
The profundity of St. Paul’s writings, the fruitfulness of his missionary labours, & the completeness of his conversion from persecutor to Apostle, fit very nicely with the belief that he was converted by Christ Himself. Your friend’s theory, OTOH, doesn’t - & the same goes for the other explanation.
I hope that helps
