On paper, Moses und Aron might seem intractable and abstract: a 12-tone score setting a libretto that meditates on God, faith, the essential inadequacy of language to express the ineffable, and a great deal more. Put it in the theatre, as Welsh National Opera has done as the first part of its ‘Faith’ mini-season, and it’s an overwhelming experience, compelling because of, rather than in spite of, its subject matter and musical methods.