Simplify the following text for a Grade 9 native English-speaking person.
Original: Sol Invictus ( `` the Unconquered Sun '' ) or , more fully , Deus Sol Invictus ( `` the Unconquered Sun God '' ) was a religious title applied to at least three distinct deities during the later Roman Empire : El Gabal , Mithras , and Sol .
Simplified: Aurelian called his sun god Sol Invictus to differentiate him from the earlier Roman god Sol .