I count religion but a childish toy And hold there is no sin but ignorance (Prologue 14-15)
Jews, Christians, Muslims—it's all the same when you're Machiavelli; religion is just a tool one uses to manipulate other people. So who agrees with Machiavelli in the play? While Barabas is touted as the main Machiavellian of the play, Ferneze is the one who really trots out religion to do his dirty work for him.
You'll make 'em friends? Are there not Jews enough in Malta, But thou must dote upon a Christian? (2.3.358-59)
Why can't you just find a nice Jewish boy? (Preferably a doctor.) In addition to demonstrating that parents haven't changed much in 500 years, Barabas is hinting at the dark future: he's already made it clear that Abigail will marry a Christian over his dead body (actually, over Mathias's dead body); just think of how he'll take the news when she becomes a nun.
A fair young maid scarce fourteen years of age, The sweetest flower in Cytherea's field, Cropped from the pleasures of the fruitful earth And strangely metamorphosed nun. (1.3.13-16)
Mathias is eyeing Abigail, who's pretending to be a nun to get back into her old house. Keep those words "strangely metamorphosed" in mind as you read on—unlike Barabas, who straight-up refuses to convert, Abigail's religious commitments are considerably more fluid. (Until they're not.)
It's no sin to deceive a Christian, For they themselves hold it a principle, Faith is not to be held with heretics; But all are heretics that are not Jews. (2.3.309-12)
Check out the way Barabas bases his Jewish practices upon Christian ones—he's adopting this Christian principle of not holding "faith" with non-Christians and then conveniently switching out their values for his.
Will Knights of Malta be in league with Turks, And buy it basely, too, for sums of gold? (2.2.28-9)
Bosco and Ferneze are making a bargain: Bosco and his Spanish navy will back up the Maltese against the Turks. Why? Because Christians should stick together. Except not really. The real reason is that Bosco thinks Malta is a good investment, so he's actually buying his own "league" for "sums of gold."
Then were my thoughts so frail and unconfirmed, And I was chained to follies of the world; But now experience purchased with grief, Has made me see the difference of things. My sinful soul, alas, hath paced too long The fatal labyrinth of misbelief, Far from the Son that gives eternal life (3.3.59-65)
Abigail take the leap to truly convert and become a nun. She has decided the leave the messy "follies of the world" and escape society. What we want to ask is: do you think that the "fatal labyrinth of misbelief" is Judaism, or the tangled web of treachery that her father is stretching across Malta?
Ithamore: Fie upon 'em, master. Will you turn Christian when holy friars turn devils and murder one another? Barabas: No, for this example I'll remain a Jew. Heaven bless me! What, a friar a murderer? When shall you see a Jew commit the like? (4.1.188-92)
Man, friars do get the fuzzy end of the lollipop in this play. Barabas has just tricked Jacomo into thinking that he's murdered his fellow friar Bernadine, saying that Christianity clearly isn't the way to go if friars kill each other. And, sure, Jacomo hasn't actually murdered Bernadine, but there are plenty of other reasons you make label him a "devil." For one, he thinks he actually is a murderer, and does his best to get away with it.
Make account of me As of thy fellow. We are villains both; Both circumcised, we hate Christians both. (2.3.213-15)
It's too bad (NOT) that Barabas isn't negotiating peace in the middle east, because it sounds like he has it all worked out: Jews and Muslins should get along, because they're both circumcised, evil, and Christian-hating. Talk about brotherly love.
Use him as if he were a Philistine: Dissemble, swear, protest, vow to love him, He is not of the seed of Abraham. (2.3.228-30)
Another example of how good behavior is only owed to people within your own community. It's okay to lie to Lodowick, Barabas tells Abigail, because he's a non-Jew (not a son of Abraham). He tells Abigail to act like the Philistine Delilah, who pretends to love the Israelite Samson and later betrays him to his death.
What, Abigail become a nun again? False and unkind! …'Tis time that it be seen into, For she that varies from me in belief Gives great presumption that she loves me not Or, loving, doth dislike of something done. (4.1.1-12)
When Barabas reads that Abigail's has become a nun, he takes it as a personal betrayal. During this time period, "unkind" would mean both "not nice" and "not the same kind." Barabas really does believe that the Christians are categorically different creatures than Jews. The fact that Abigail wants to join them indicates that she is of their "kind," and not his.