It's okay, for example, to cite religious leaders if they are talking about their religious beliefs. "We believe in reincarnation." That is a verifiable fact.
It can be okay to make religious beliefs the object of academic inquiry. "How does a person's belief in reincarnation affect the decisions he or she makes in life?" Studies can be constructed and, through measurement and observation, including observations made by religious leaders, hypotheses can be verified.
But it's not okay to argue in an academic paper that because your religious leader says it is so, reincarnation is a real phenomenon. At this point, we just don't have a way to verify it independently. (Maybe in another life we will.)
The new twist today? Usually I'm talking with LDS students about LDS leaders and scripture. Tonight I enjoyed talking with a Catholic student who had written in her research proposal that she was planning to consult her priest about her topic.