The conflict between Antonio and Shylock arises partly from religion and partly from economic causes. Antonio wants to borrow money from Shylock, who is employed as a moneylender. While he needs Shylock, Antonio also curses him. In addition, Shylock is Jewish, and Antonio is Christian, and anti-semitism on Antonio's part is a reason behind the feud between them. Shylock says to Antonio:
"Signior Antonio, many a time and oft/In the Rialto you have rated me/ About my moneys and my usances:/ Still have I borne it with a patient shrug, /For sufferance is the badge of all our tribe. /You call me misbeliever, cut-throat dog,/ And spit upon my Jewish gaberdine, /And all for use of that which is mine own."
In other words, Antonio has cursed Shylock for being a money lender, as "usances" refers to the practice of lending money and charging interest. Shylock believes he has handled this abuse with patience, as Jews suffer patiently and often at the hands of Christians who mock them. Antonio has abused Shylock and even spat on him because Shylock lends out money that belongs to him. However, Antonio now needs money from Shylock, which causes their conflict to become even more bitter.
Many critics, including the preeminent Shakespeare scholar Harold Bloom, have called the play anti-semitic. In his 1998 book Shakespeare and the Invention of the Human, Bloom charged that the play resulted in many centuries of anti-semitism in Europe, ending in the Holocaust. See the link to the Smithsonian article below for more information about the problematic nature of the play and the debate about whether it is anti-semitic in nature. There is no doubt that religion is at the core of the conflict between Antonio and Shylock.