Quote 1
FAUSTUS
Jerome's Bible, Faustus; view it well.
[Reads] "Stipendium peccati mors est." Ha!
Stipendium, etc. The reward of sin is death. That's hard.
[Reads.] 'Si peccasse negamus, fallimur,
Et nulla est in nobis veritas.'
If we say that we have no sin,
We deceive ourselves, and there is no truth in us. (1.1.36-42)
Here, Faustus reads lines from the Bible, specifically Romans 6:23, "The wages of sin is death," and 1 John 1:8. But Faustus isn't exactly being a thorough reader here. Romans 6:23 goes on to say, "but the gift of God is eternal life through Jesus Christ our Lord," while 1 John 1:8 reads, "If we confess our sins, he is faithful and just to forgive our sins and to cleanse us from all unrighteousness." Considering the fact that Faustus is such a renowned scholar, it's hard to believe that he doesn't know the rest of these lines. Maybe he's just already made his choice, so he's reading the Bible in a way that validates it. It's clever, sure, but it doesn't exactly help him in the end.
Quote 2
FAUSTUS
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! (1.1.42-46)
The way Faustus sees it, the Bible tells him that he is damned to hell no matter what he does (never mind the fact that he didn't read the whole thing). In a weird way, it makes sense then, that he rejects the study of theology. After all, wouldn't it be worthless if, no matter how much you study it, it doesn't buy you a ticket to heaven. And to be fair, however incomplete it is, Faustus's interpretation of these Bible verses comes close to the Calvinist doctrine of Predestination—that a man's fate as saved or damned is set in stone long before he's even born. The only difference is that here, Faustus doesn't think anyone can be saved.
Quote 3
FAUSTUS
Seeing Faustus hath incurred eternal death
By desperate thoughts against Jove's deity,
Say, he surrenders up to him his soul. (1.3.86-88)
Faustus's logic in surrendering his soul to the devil is similar to his thinking when he turned to magic: "I am already damned, so why not go whole hog?" In other words, Faustus has already blasphemed against God, and has therefore lost all hope of heaven. There's no turning back now.