Ezekiel 36:33
Thus saith the Lord GOD; In the day that I shall have cleansed you from all your iniquities I will also cause you to dwell in the cities, and the wastes shall be builded. Thus saith the Lord GOD; In the day that I shall have cleansed you from all your iniquities I will also cause you to dwell in the cities, and the wastes shall be builded.
On the day God cleanses his people from all their iniquities, he causes them to dwell in the cities and has the ruined places rebuilt. Both the cleansing and the rebuilding are his own doing and not anything Israel earns, showing mercy that repairs what sin left desolate.
Around that promise the desolate land is shown becoming fruitful and inhabited again, even like the garden of Eden. In that restored condition his people serve as a sign to the surrounding nations that the Lord himself has rebuilt the ruins and planted what was desolate.