I will have mercy on whom I have mercy.
These words are sweet and remarkable. These words come from God, and they are in response to post-Exodus Israel making a golden calf idol and worshipping it with feasting and sexual immorality. In this incident Israel shows itself to be no less idolatrous than Pharaoh and the Egyptians. Yet, God will show mercy to Israel.
God is clear that his mercy is pure and comes from him alone, and is not because of any obligation in Israel's previous obedience in following God out of Egypt. It is pure mercy. It is shocking mercy. It is unimaginable mercy. It is prophetic mercy.
How is it prophetic? It is prophetic in that God was passing over Israel's idolatry in the moment, because millennia later Jesus would die as a sin-bearer for Israel's sins (Romans 3.21-26). For God to be just in showing mercy to Israel, the sins had to be atoned for. Jesus does that atoning work in his life, death and resurrection.
This is true for us as well today. Because of Jesus' work on the cross, God shows us pure mercy, a mercy that comes from his heart, that he isn't obligated to show us—but he does. A mercy that is free for us, but costly for God. A mercy that we can receive but not earn. In Christ you are an object of mercy and vessel of God's mercy. As objects of mercy, as vessels of mercy, delight in God's mercy and give your life to showing mercy to others. Mercy triumphs over judgment (James 2.13).