You don't have to be religious to enjoy the tranquil atmosphere of Nancy Cathedral. Inquire when the next mass starts or when it's open for general viewing.
This principal church is one of the best free attractions in Saint Nicolas - Charles III - Ville vieille - Trois Maisons - Léopold. If you're in the mood to adventure beyond Nancy Cathedral, also stop by Place Stanislas, Musee de lEcole de Nancy and Musee des Beaux-arts.