Shakespeare On Flawed Human Justice.

"In the course of justice, none of us
Should see salvation: we do pray for mercy,—"

—"The Merchant of Venice",
Act IV, Scene 1.

What Portia is trying to say, in "none of us should see salvation" is that Shylock seeking answers and solutions to deeper personal questions, like should Christians be mean to Jews. But like in that Lord Halifax quote about being hanged for stealing horses, human justice is ultimately for unresolvable disputes. We (i.e., all of us) do pray for mercy, because human justice, unlike God's, is flawed and may fail sometimes. That's the other reason why it should only be a last resort.