Faustus rejects God, aspiring to become 'a mighty god' himself. He declares, 'There is no chief but only Belzebub', but eventually tries, and fails, to repent.
Doctor Faustus: AS & A2
The play's language makes a clear distinction. Earth is seen as the domain of the devil; the soul is associated with air and the heavens.
Faustus wishes to master magic, but his blasphemy summons Mephastophilis. Faustus's power lies solely in Mephastophilis. Devils can change appearances, but not substance.
Faustus deceives himself into rejecting Christianity by false logic. Mephastophilis and Lucifer deal honestly with Faustus but help him to trick others with false appearances.
Faustus's search for knowledge could be seen as heroic – he strives to live beyond the limits of human endeavour – but it also proves to be his downfall.