Have you ever seen those cartoons where a generally a-okay character is tempted to do something really bad? Suddenly, a little red dude holding a pitchfork appears on one shoulder, and a serene-looking figure with wings and a halo, clothed all in white, appears on the other.
Well, if you've seen that, then you pretty much know the drill with the Good and Bad Angels. They appear every time Faustus begins contemplating the state of his mortal soul. The Good Angel's goal is to convince Faustus to abandon his sins and return to God, while the Bad Angel tries to get the scholar to continue in his pursuit of magic and so remain loyal to the devil. Choices, choices.
But here's the real scoop: the Good and Bad Angels are actually a literary device called allegory. That means they're the personification of abstract concepts in a concrete form. Fancy, right? Here, the Good angel represents Faustus's desire to repent, and the Bad Angel, his desire to keep right on sinning. As they battle back and forth, so Faustus battles within himself. In short, it just might all be in his head.
See, Faustus remains really divided throughout the whole play. On the one hand, he's pretty sure that he was damned to hell the minute he even contemplated becoming the devil's servant. This is what the Bad Angel wants him to believe.
But, on the other hand, isn't there such a thing as repentance, feeling so bad about your sins that God is willing to forgive you for them? That's what the Good Angel, among other characters, keeps trying to convince Faustus of.
In the end (spoiler alert!), the Bad Angel wins. But if the Bad Angel is really just a personification of part of Faustus's mind, then doesn't this really mean that one half of Faustus's divided mind has triumphed over the other?