Quote 7
FAUSTUS
If we say that we have no sin,
We deceive ourselves, and there is no truth in us.
Why, then, belike we must sin
And so consequently die.
Ay, we must die an everlasting death.
What doctrine call you this? Che serà, serà?
What will be, shall be? Divinity, adieu! (1.1.41-47)
Shmoop calls this the Doris Day doctrine. But here's the thing. Faustus doesn't just think that mankind is predestined to sin, and is therefore headed to hell. He also thinks that, because of this, studying religion has no point. This tells us that Faustus is not interested in knowledge for its own sake—only for how it can benefit him. But the joke's on Faustus, because if he had studied religion, he probably wouldn't be in this predicament.
Quote 8
FAUSTUS
Stay, Mephistopheles, and tell me
What good will my soul do thy lord?
MEPHISTOPHELES
Enlarge his kingdom.
FAUSTUS
Is that the reason why he tempts us thus?
MEPHISTOPHELES
Solamen miseris socios habuisse doloris. (2.1.38-41)
Mephistopheles's Latin response to Faustus's question translates into "to the unhappy it is a comfort to have had company in misery." (In other words, "misery loves company.") This is basically a warning from Mephistopheles to Faustus to turn back from his intended course of action, since it implies that hell is miserable. But Faustus ignores it. He's really good at ignoring people.
Quote 9
FAUSTUS
Now tell me who made the world?
MEPHISTOPHELES
I will not.
FAUSTUS
Sweet Mephistopheles, tell me.
MEPHISTOPHELES
Move me not, Faustus.
FAUSTUS
Villain, have I not bound thee to tell me anything?
MEPHISTOPHELES
Ay, that is not against our kingdom;
This is. Thou art damned; think thou of hell.
FAUSTUS
Think, Faustus, upon God that made the world. (2.3.66-74)
Why is the answer to Faustus's question so threatening to Mephistopheles? Maybe because the answer—that God made the world—is just too good. After all, if God made the world, then God must be awesome, and Mephistopheles only wants to talk about the bad stuff. Acknowledging the goodness of god would threaten Mephistopheles and Lucifer's hold upon Faustus's soul.