Most oratorios have a biblical themes, but a number of composers, notably George Frideric Handel, wrote a number of works based on themes from Greek and Roman mythology. These hybrid works (part opera, part oratorio) are customarily referred to as secular oratorios.
Oratorios (like operas) can contain several elements:Messiah by Händel. It contains the well known halelujah-chorus. Others well known oratorios include the Christmas Oratorio by Johann Sebastian Bach, The Creation by Joseph Haydn, and Mendelssohn's Elijah oratorio.