But Israel’s sins will be forgiven only when the stones of pagan altars are ground up like chalk, and no more incense altars or symbols of the goddess Asherah are left.” (Isaiah 27:9, GNB)

The issue here is not that God is waiting for Israel to work out her repentance before he forgives them.  The issue is that something was standing in the way of them being forgiven; something made forgiveness impossible.  They were still in their sin of idolatry.

In order to receive forgiveness, the Israelites had to turn away from idolatry, otherwise what would be the point.  They could not come back to covenant with God and continue on in their idolatry.

The people would not leave their idolatry and so God undertakes to force them to leave it.  Thus the reason for the destruction of Jerusalem and exile in Babylon for 70 years.  Whatever problems the Jews had in the post-exilic period, worshiping idols was not one of those problems.  God had ridden them of their idolatry.  This was a sign of grace.

One wonders how this verse would read if it was written to the American church today.  We are not far removed from the Israelites just before the destruction of Jerusalem.  We are comfortably ensconced in materialism.  We are lazy.  We lack evangelistic fervor.  We lack love for each other and for our communities.  I could go on, but you get the point.