Hopefully making theology simpler to understand! (2 Corinthians 1:12)

Of the increase of his government and of peace there will be no end, on the throne of David and over his kingdom, to establish it and to uphold it with justice and with righteousness from this time forth and forevermore. The zeal of the Lord of hosts will do this.
Isaiah 9:7, ESV

God has come to us and saved us, and when He returns His Kingdom shall be the only kingdom full of His saved saints.