Simplify the following text for a Grade 9 native English-speaking person.
Original: It is well documented in historical religions of Classical Antiquity , especially Greek polytheism and Roman polytheism , and after the decline of classical polytheism in tribal religions such as Germanic polytheism or Slavic polytheism .
Simplified: Polytheism means believing in many gods . A person that believes in polytheism is called a polytheist .