Is it permissible for Catholics to address leaders of other faiths as ‘‘Your/His Holiness’’ if that is the title used for them by members of their faith? For example, the Dalai Lama of the Tibetan Buddhists or Syedna Mohammed Burhanuddin Saheb of the Dawoodi Bohra Muslims. I am talking of course about legitimate religious leaders, not cult founders.
So long as there does not appear to be a danger of undue scandal – for example, giving the impression to outsiders that one is a follower of a non-Catholic religious leader – there is nothing inherently wrong with using the term “Your Holiness” for those religious leaders other than the pope who are given that title as an honorific. If one fears that scandal may be given, it is, of course, possible to use President George W. Bush’s solution and call the religious leader “sir.”