Follow that with an assertion that individuals who are not themselves spiritual, most of which freely admit having little or no education on or experience with spiritual matters are qualified in any way to speak on spiritual matters.
I'd advise you to read The Courtier's Reply. Not that it covers everything you mentioned, but it is somewhat relevant.
People are much more likely to criticize what they see constantly around them. In a good portion of the world, Christianity is far more visible than Buddhism or Taoism. Ergo, Christianity gets criticized.