In the play The Merchant of Venice, how does mercy or charity relate to the central concerns?

Mercy, charity, and the Christian faith all play huge thematic roles in this play. In the start of the play, Antonio lends money to his friend Bassanio in an act of charitable friendship. He does this to help his young friend court Portia, the woman Basannio later marries. Antonio is a wealthy merchant. Since most of his wealth is invested in ships, however, he must take a loan from the moneylender Shylock in order to get the funds. Shylock gives him the money with the hopes of making a profit when Antonio's ships return. Bad lucks eventually leaves Antonio without the money he needs. This leaves Shylock in a position where he may extract a pound of flesh from Antonio, which would of course kill the merchant.

To save Antonio, Portia dresses up as a judge and gives a speech on the value of mercy. She argues that Shylock is wrong to take vengeance. She claims that "the quality of mercy is not strained," and argues that forgiveness is a higher value than justice. This aligns with the teaching of the Christian New Testament, in which Jesus also preaches forgiveness over justice. Modern scholars often find this solution disturbing, however, since Shylock, the jewish character, is not only deprived of his payment, but also forced to convert to Christianity. This does not appear especially merciful for a man of his faith. Many consider this an act of injustice rather than an act of true mercy.