Israel had fallen to such an extent that Isaiah claims Abraham would not recognize their faith and Jacob (renamed to Israel) would not even know his namesake. They had completely abandoned their heritage of faith. They were a centerless people.
Despite their current spiritual condition, they could still repent and appeal to the God Who was still their Lord, their Father, and their Redeemer. Those three realities could strengthen a broken people. It could put the entire nation back together again. It could restore a people.
To be in surrender to The Lord was the first step toward reclaiming their identity as God's unique people. It meant worship and submission. It would keep them focused on God's glory and their position with Him.
To be in relationship with God as Father made the entire thing personal. It made it meaningful. It put God as close to hearts as the nearest family. And this was ultimately what Jesus would bring to the world... an intimacy with God as our Father.
And to know God as Redeemer brought salvation. God delivers from sin's slavery. He lays out a new life before His redeemed people now for His purposes. Redemption gave them new life worth celebrating. God makes all things new.