Barabas never met an aphorism he didn't like. He's quick to spout off a Latin quote or a pithy saying to make his point. So what do all of these sayings do, exactly?
Well, Shakespeare scholar Stephen Greenblatt makes the point in his essay "Marlowe, Marx and Anti-Semitism"(source) that Barabas may not be so different from the rest of the Maltese as the play makes him out to be. All these conventional saying actually place him at the heart of Maltese society.
His aphorisms also make him somehow less of a person. Instead of using his own words to make a point, Barabas usually falls back a proverb, like "faith is not to be held with heretics" (2.3.311). The result? We can't actually use what he says to figure out who this guy is. Barabas is a difficult guy to understand because it's so hard to understand what parts of him are him and what parts are literary and cultural stereotype—and whether or not there's even a difference.
We cover this in Setting, but just to recap: place is a big deal in this play. Think of the slave market, and the roles everyone takes on there: A Jewish man is buying a Turkish slave who's being sold by the Spanish Christians.
Or thing of Barabas's house: it starts out as a symbol of his power and success; it becomes a nunnery (the symbol of all the forces that oppose him—not just Christianity, but Christian hypocrisy); and then Barabas reconquers it when he poisons all the nuns. (Sorry, ladies.)
So, location tells us a lot about social dynamics. And keep your eye on the big picture: the point when Barabas find himself on the other side of the city walls and vows revenge on Malta is a big turning point in the play.
Switching careers can mean a midlife crisis. In The Jew of Malta, it's even more important: it means major plot shifts and changes in characterization.
Take Barabas. He starts off as a merchant, transforms into a revenger and murder, becomes the Governor of Malta … and then dies.
Or Abigail. She starts off as a dutiful daughter (yep, it's a profession), and then becomes a nun: a signal that the things that Barabas has done to trigger her conversion are way out of line.
Why do occupations matters so much? Because they're "professions." Profession is a slippery, flexible concept—from the Governor to the friars to the prostitutes, everyone's willing to play a little dirty. In fact, one of the more honest moments of the play is when the slave Ithamore promises to Barabas that his profession is "what you please."