Who is the Christian here and how will he play the knave?in act 2 scene 3

That would be Launcelot who says,

"Adieu! Tears exhibit my tongue. Most beautiful pagan, most sweet Jew! If a Christian do not play the knave and get thee, I am much deceived. But adieu. These foolish drops do something drown my manly spirit. Adieu."

The Christian might be Lorenzo.