The verse for today is a quote taken from Exodus 33:19. In order to understand the thrust of what Paul is saying you need to read the context of Exodus 33:19. A good place to start is chapter 32, verse 1.
Paul quoted Exodus 33:19 to show that God’s extends mercy according to His sovereign choice. It is in reference to Israel’s idolatry while Moses was on Mount Sinai receiving the Ten Commandments. The whole nation deserved to be destroyed according to God’s justice. However, only 3,000 people died because of God’s mercy. They died not because they were more wicked or less godly. Every Israelite stood guilty before a just God and deserved to die. God extended mercy to the rest of the nation.
Now if we approach the killing of these 3,000 Jews from a human viewpoint, our response would be, “Oh my goodness! How could God kill 3,000 Jews just like that?” However, if we respond from a Biblical view, our response would be, “Oh my! How could God not kill all of the Jews? What a merciful God!”
Because all of humanity is sinful and deserves God’s condemnation, no person is wronged or treated unjustly if God chooses to allow him or her to receive the just punishment for sin. That is justice. His mercy toward any person is purely by His grace. The issue in such matters is God’s sovereign choice. As the sovereign God, He declares, “I will mercy on whom I have mercy and I will have compassion on whom I have compassion.”