In Act I, scene three, of William Shakespeare's The Merchant of Venice, Shylock refuses to eat with Antonio and Bassanio. In lines 29-34, Shylock defines his reasoning:
Yes, to smell pork; to eat of the habitation which your
prophet, the Nazarite, conjured the devil into! I will buy
with you, sell with you, talk with you, walk with you, and
so following; but I will not eat with you, drink with you,
nor pray with you.
Shylock's refusal to eat, drink, or pray with Bassanio and Antonio exists because of his own religious ideologies (beliefs). Shylock is Jewish. According to Jewish law, pigs are not allowed to be eaten because they are not kosher. Animals which are kosher are those which are ruminant (chew the cud- to chew food, swallow, bring back into the mouth, and chew again) and have split hooves. Now, although pigs have split hooves, they do not chew their cud and are considered unclean to eat or touch.
In the end, Shylock does not agree with the Christian beliefs of Bassanio and Antonio. Since Jewish, he separates himself from those who do not follow his own religious beliefs.
I really hope this helps. I tried breaking down the reasoning behind Shylock's refusal completely (to help with your understanding).