What city is Isaiah 1:21 talking about when it says the faithful city became a harlot?

Isaiah 1:21 refers to Jerusalem. The faithful city that became a harlot is Jerusalem, which had once been full of judgment and righteousness but had turned to murderers and wickedness. This is the city the prophet Isaiah was addressing when he rebuked the nation of Israel for its sin.

Les explains that Isaiah is coming down on the nation of Israel because of all their sin and wickedness. The faithful city that became a harlot is Jerusalem. At one time it had been a righteous city, and the Jewish people were obedient to the Mosaic Law and the system. But they had become so wicked that the city once full of righteousness was now full of murderers. Les also points to Isaiah 1:22 and the word translated corrupt in 2 Corinthians 2:17.

Isaiah 1:21

How is the faithful city become an harlot! it was full of judgment; righteousness lodged in it; but now murderers.

What was Jerusalem like before the judgment described in Isaiah 1?

Before the judgment, Jerusalem was a faithful city, full of judgment and righteousness; righteousness lodged in it. The Jewish people were obedient to the Mosaic Law and its system. Isaiah contrasts this former righteous condition with its later state as a harlot and full of murderers.