How do we know that God's mercy is a key aspect of His glory?

In God's reply to Moses in Exodus 33:19, He declares, 'I will be gracious to whom I will be gracious, and I will show mercy on whom I will show mercy.' This underscores that His glory is not only seen in His majesty but greatly in His mercy and grace. Matthew Henry noted that we know God more by the glory of His mercy than by the glory of His majesty, revealing that God's merciful nature is an essential truth that His people must understand. The apostle Paul reiterates this in Romans 9:15, affirming that it is God's sovereign choice to show mercy, illustrating His character.