How do we know God has mercy on whom He chooses?

Romans 9:15 provides clarity on God's mercy: 'I will have mercy on whom I will have mercy.' This statement illustrates the sovereignty of God in determining whom He chooses to save. It emphasizes that salvation is not based on human effort ('not of him that willeth, nor of him that runneth') but entirely on God's will and purpose. This doctrine of sovereign grace underlines the importance of recognizing that God's decisions are rooted in His justice and righteousness, leading to a deep assurance for those whom He has called, as salvation is rooted in His sovereign mercy.