What does Isaiah 1:21 mean?

Isaiah begins a new section here, returning to the Lord's lawsuit against Israel. He adds charges to those stated before (Isaiah 1:2–4) to show how the people of Judah have broken their covenant with the Lord.

He is comparing the city of Jerusalem during Israel's glory days under David and Solomon to the state of the great city during present time. Jerusalem was once known as a city of people faithful to the Lord. The capital of Israel was known as a place of justice, where the righteous sought to reward those who did right and condemn those who did evil.

Here, Isaiah uses the uncomfortable, challenging parallel between false worship and sexual infidelity. In a spiritual way, the faithful city has become "a whore." The covenant relationship between God and Israel is often compared to a marriage in the Bible, especially by the prophets Jeremiah, Hosea, and Isaiah (Isaiah 54:5). Jerusalem, the once faithful wife, has become a prostitute. In other words, the people have cheated on the Lord. They are committing adultery by worshiping false gods Instead of the Lord, in hopes to get what they want from these false gods.

Also, instead of being famous for her justice, Jerusalem is now known as a place where murderers walk free. This may refer to those who take advantage of the poor and powerless to the point where they die in destitution.