Quote 1
MEPHISTOPHELES
For, when we hear one rack the name of God,
Abjure the Scriptures and his Savior Christ,
We fly in hope to get his glorious soul;
Nor will we come unless he use such means
Whereby he is in danger to be damned. (1.3.45-49)
The idea that swearing—taking God's name in vain, or cursing God—draws devils around you who will to try to win your soul for the Dark Side, is not a new one. In fact, it dates back to medieval times (no, not the restaurant). Back then, folks thought that a person always opens a space in his heart for the devil when he sins, but by swearing, he announces it to the world, basically advertising to evil spirits that his soul is theirs for the taking.
Quote 2
MEPHISTOPHELES
Hell hath no limits, nor is circumscribed
In one self place, but where we are is hell,
And where hell is, there must we ever be. (2.1.118-120)
Ever read Paradise Lost? In that piece, Satan declares, "Myself am hell." Like that declaration, Mephistopheles's description moves close to defining hell not as a place, but as a state of the soul. Those souls that are separated from God by their sins are in hell no matter what physical place they are in. It's everywhere.
Quote 3
MEPHISTOPHELES
But think'st thou heaven is such a glorious thing?
I tell thee, Faustus, it is not half so fair
As thou, or any man that breathe on earth.
FAUSTUS
How prov'st thou that?
MEPHISTOPHELES
'Twas made for man; then he's more excellent. (2.3.5-9)
Maybe we're missing something here, but why does it necessarily follow that heaven must be less beautiful than man just because it was made for him? This is probably an example of the twisted logic the devils often use to get a hold on Faustus's soul. Of course it totally works. See, Faustus? You should've stayed in school.