There are two dimentions.  The assiah where humans live and Gehanna where demons are and ruled by satan. 

There are those called Exorcists, people who train to destroy demons who act in a damaging manner in Assiah.

When the protagonist named Rin witness how satan killed his adoptive father,  he began to learn things that he himself doesn't want to believe. Including being born as the son of satan and his twin brother as a exorcist. 

<3 

You'll see how determined rin to prove that he will not cause any harm specially to his so called friends even though his Satan's son.