What does Isaiah 1:25 mean?

Isaiah has described the catastrophic spiritual state of God's people, even within the borders of the once faithful city of Jerusalem (Isaiah 1:21). Instead of being like pure silver (Isaiah 1:22) Jerusalem has become like diluted silver full of impurities known as "dross." This is because of the rebellion and corruption among God's people.

The Lord has declared His intention to change this corruption. He has said that He will avenge Himself among His people. (Isaiah 1:24) Now He says that will act against them, removing everyone who is not faithful to Him. Describing a painful process, the Lord announces that He will remove all impurities. The metaphor of silver and dross hinges on this refining technique: heating silver until the impure elements are melted out or burned away.

This painful process points to God's coming judgment in the form of the Babylonian invasion of Judah and the exile from Israel. As well as the time after when a remnant of the faithful will be restored (Isaiah 1:26) .