Saint Francois d'Assise, Olivier Messiaen's only operatic work, received its world premiere in Paris in 1983. It has rarely been performed since, partly because of the sheer scope and audacity of the project, but also because of its subject matter--faith itself. This fall, the San Francisco Opera, newly directed by Pamela Rosenberg, gave the opera its U.S. premiere in its namesake city. It was a brilliant gamble, possibly opening a new operatic door in America. This is an opera unlike any other--an unabashed paean to music, to nature, and to the mystical path to joy seen in the figure of Francis (sung movingly by baritone Willard White).