Being a hypocrite is pretending to have morals or virtues one does not, in reality, possess.
Portia, even if unwittingly, displays hypocrisy when she tries to persuade Shylock to show mercy towards Antonio. She tells him that mercy is "an attribute of God himself," but does not in the end lean toward mercy for Shylock or try to persuade Antonio to practice mercy. Christians, according to Portia's speech, are implicitly superior to Jews because they put mercy ahead of justice. However, the Christians in the courtroom are willing to treat Shylock quite harshly. Portia is willing to have Shylock forfeit all the money Antonio owes him, and Antonio is hardly less harsh, offering Shylock part of the money only if he makes Lorenzo, the Christian who eloped with his daughter, his heir and himself converts to Christianity, an idea that is odious to Shylock.
Further, even before getting to the courtroom, Antonio's behavior is hypocritical for one who asserts superiority on the basis of being a Christian: Antonio is very unkind to Shylock, showing unChristian behavior in the way he insults him, spits on him, and even strikes him. He also tells Shylock he is not likely to stop this behavior.
Although Christianity preaches both mercy and love of one's enemy, neither Portia nor Antonio seem able to extend that concept to their treatment of Jews. Therefore, both are hypocrites, untrue to their professed beliefs.