Sufism
Sufi practices are usually defined within the basic tenets of Islam, the Sharia (or Shariah)  although some orders include singing and dancing in the love of God, something some Muslims frown on. Hence some Muslims of a more orthodox mind-set believe that any actions outside the scope of Sharia, or Sufi activities practised by non-Muslims are 'wrong'. However the underlying message and aim of the quest for the love of God is true for all humanity. For a non-Muslim the most important ideas in Sufism are love for fellow human beings (which reflects one's love of God) and living ones life in such a way where all actions are against the self. This automatically starts him on the path of fanaa. In terms of discipleship and training, almost all the major Sufi orders are themselves quite willing to take students who are not Muslims.