Premiered in 1846, “Elijah” is based on the first and second book of Kings in the Old Testament, in which the prophet Elijah deals with drought, vengeance and idolatry before ascending to heaven on a chariot of flames. If it sounds like the makings of an opera, you’re right, but church prohibitions on performances of opera during the season of Lent turned Mendelssohn toward the oratorical form.