Note: Before we jump into discussing those pesky sins, we want to tell you that there are two other allegories in this play—the Good and Bad Angels, and the Old Man. You can find out more about them in our section on "Characters."
The Seven Deadly Sins that Mephistopheles's devil friends conjure to amuse Faustus are an allegory in the purest sense of the term.
An allegory is an abstract concept that appears in a material, concrete form. And in this case, the seven deadly sins (which separate a person from God forever if they're not repented) appear as actual people.
In front of Faustus, Pride, Covetousness (Greed), Envy, Wrath, Gluttony, Sloth, and Lechery (Lust) march in the weirdest parade that ever paraded. They describe their parentage—that is, where and whom they came from—and defining characteristics.
Sometimes, their parentage is a metaphor for the way the sin takes root in the soul. Covetousness, for example, says he is "begotten of an old churl in a leather bag" (2.3.120-121), which probably refers to a miser and the sack of gold that's more important to him than anything else. Pride, on the other hand, "disdain[s] to have any parents," just as people who have too much pride refuse to recognize any authority other than their own.
But before you go giving Marlowe props for being so clever, you should know that he was not the first person to personify the Seven Deadly Sins. In fact, a boatload of people did so long before he was even born.
Medieval drama had a long tradition of representing the Seven Deadly Sins as people, so when Doctor Faustus was first performed, the Sins would probably have come onstage in immediately recognizable costumes. The audience would have known exactly what was going on.
And even we modern folks are in on the joke. The things the Sins tell Faustus about themselves are exactly what we'd expect: Gluttony, the sin of overindulgence in food and drink, complains that his parents left him "only" enough money for thirty meals and ten snacks a day, while Sloth, the sin of laziness, doesn't even have enough energy to describe himself (okay, that's pretty funny, Marlowe).
In the medieval tradition of allegory, a character's relationship with the Sins tells us which side he's on—God's, or the devil's. Three guesses where Faustus falls. He just laughs about them, which tells us not only that he's on the side of the devil, but also that he's there because he doesn't take sin as seriously as he should. Not cool, dude.