In The Merchant of Venice, a play of dichotomies, Portia may be seen as representing the theme of mercy as is eloquently attested by her "Mercy" speech (IV.i.184-197). In dichotomy to this, Shylock may be seen as representing the opposing theme of revenge, or vengeance, as is attested by his loan contract and legal appeal for the one pound of flesh Antonio foolheartedly promises as security against Bassanio's loan. Some might see Shakespeare's exploration of the theme of mercy versus vengeance as tongue-in-cheek, in other words, as an ironic perhaps mocking exploration in that Shakespeare clearly presents a good case for Shylock to desire revenge based on his rendering of grievances against all of Antonio's base behaviors and based on the obvious shortcoming in Portia's defense of mercy as it is withheld from Shylock, who is stripped of everything including his religious and ethnic identity.
Related to a second theme, Bassanio, Antonio and Portia may be seen as representing the theme of outward show to impress others, or appearances for the sake of impressing others. Bassanio wants a loan of a substantial amount of money in order to impress Portia with the appearance of wealth in order to attempt to win her hand (and wealth) in marriage. To impress by appearances, Antonio brags to Solanio and Salerio that his livelihood and fortune do not depend upon the success or failure of one shipping venture, then confesses to Bassanio that he can't give him a loan because everything he has is tied up in his ships and presently at sea. Portia and Nerissa impress by appearances when they impersonate legal personages and speak up for Antonio, though as said before, Portia's eloquence and mercy (perhaps appearance of mercy) extends only as far as Bassanio and Antonio.
One theme, religious discourse and reconciliation, is partly conveyed by the beginning and end of Shylock's much noted speech: "To bait fish withal..............The villainy you teach me, I will execute, and it shall go hard but I will better the instruction"(3.2). The ending recalls Portia's "It is a good divine that follows his own instructions; I can easier teach twenty what were good to be done........"(1.2). The beginning recalls Gratiano's "But fish not with this melancholy bait"(1.1). In turn we have the first and last lines of the first conversation in ROMEO and JULIET: "Gregory, on my word,................'t is well thou art not fish; if thou hadst, thou hadst been poor John." In due course, one might recall instructions to the disciples in the Gospels and the first line from The Gospel of John: "In the beginning was the Word." The "Word," as they say, is in part a reference to a classical Greek concept associated with philosopher Heraclitus. I don't know if this is the same Heraclitus Professor Halio tells us is the "weeping philosopher" Portia refers to(1.2). In the middle of Sampson and Gregory's chat we find "'T is all one." That this an allusion at once to GENESIS 1:27 and DEUTERONOMY 6:4, I would argue, is plain enough. Shakespeare would later write in MACBETH, the so-called Scottish play, "Such welcome and unwelcome things at once / 'T is hard to reconcile"(MAC4.3).