The quality of mercy is not strained;
It droppeth as the gentle rain from heaven
Upon the place beneath. It is twice blest;
It blesseth him that gives and him that takes:
Tis mightiest in the mightiest; it becomes
The throned monarch better than his crown:
His sceptre shows the force of temporal power,
The attribute to awe and majesty,
Wherein doth sit the dread and fear of kings;
But mercy is above this sceptred sway;
It is enthroned in the hearts of kings,
It is an attribute to God himself;
And earthly power doth then show likest God's
When mercy seasons justice. Therefore, Jew,
Though justice be thy plea, consider this,
While both mercy and power are wielded by kings, mercy goes even beyond the official authority of the position. 
 The phrase “sceptred sway” here means “the official use of authority or power” – which “mercy is above.” 
 “Mercy”, instead of taking on an authoritative power, takes on the connotation of godly power – “an attribute to God himself.” God allots humans mercy, and so kings must aspire to do the same, Portia argues, attempting to glorify mercy as the act of God. 
 Moreover, the king, as the anointed representative of God on earth, was at a midpoint between lowly humans and the divine presence of God.  His scepter and throne and crown are tangible symbols of the power he holds over mortals on earth, but the real power he holds is divine power from God, particularly the power to show mercy, forgiveness of sin, redemption.  Mercy, because it is “an attribute to God himself” is more powerful than his “sceptered sway.” 
 It also needs to be noted that the idea of forgiveness of sin is (in this context) a Christian concept.  The Jewish faith values the atonement for sin on the highest of the holidays, Yom Kippur, but it is a personal and specific atonement that one undertakes through apology, restitution, fasting, prayer, and other observances.  To the best of my knowledge, one’s sins are  atoned  for, but that is not the same as being  forgiven.   Portia is approaching Shylock, therefore, with a philosophy that does not completely intersect his own.  She is preaching Christian forgiveness to a person who believes in atonement, restitution, a payment of debt.  This is, in many important ways, a failure to understand her audience and his worldview.
The Quality of Mercy