The only reference in the play about what Shylock shall do to a Christian is when he refers to Antonio. He mentions that he will take revenge on him for the injustices he feels Antonio had done to him. He feels that he is justified in his action against the Christian since Antonio had spat on him, kicked him as if he were a dog, called him a dog and publicly denounced his moneylending practices.
Furthermore, Antonio also lends out money free of interest, which has a detrimental effect on his business, since Antonio's actions bring down the rate of interest. Finally, he says, he will hurt Antonio by taking a pound of his flesh not only because he had forfeited on his promise to repay his debt, but also because he hates Antonio. He mentions as much to the duke during the trial:
... So can I give no reason, nor I will not,
More than a lodged hate and a certain loathing
I bear Antonio, that I follow thus
A losing suit against him.
Earlier, in Act One, when Bassanio approached him for the loan with Antonio as surety, Shylock expressed confidence that he finds Antonio sufficient, i.e. that he is of the means to settle the debt. He requests to speak to Antonio. Bassanio then invites him to dine with them so that they may converse. Shylock responds:
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. What
news on the Rialto?
In this extract, he clearly states four things that he will do with a Christian: he will buy, sell, talk and walk with a Christian but he refuses to eat, drink or pray with one. It is clear that Shylock detests Christians and does not want to associate with them, except on a very rudimentary level. He is, therefore, more than prepared to discuss business with Christians, but deems it inconceivable that he will form any deeper relationship than that.