The Kingdom Of God in Four Stages

The Kingdom Of God goes through four stages.

Stage 1: Israel
And I will put enmity between you and the woman, and between your offspring and hers; he will crush your head, and you will strike his heel." (Genesis 3:15 NIV)
This stage began from the first prophecy by God to the serpent through His promise to Abraham, Isaac and Jacob. It officially started with establishment of Israel through Moses.
The end of this stage came when Judah was destroyed by Babylon. At that moment, it seems that the God of Abraham had lost. Marduk (i.e. Baal), the chief god of Babylon had won.

Stage 2: Unobserved Growth (now)
Being asked by the Pharisees when the kingdom of God would come, he answered them, "The kingdom of God is not coming in ways that can be observed, nor will they say, 'Look, here it is!' or 'There!' for behold, the kingdom of God is in the midst of you." (Luke 17:20-21 ESV)
The kingdom of God in the current age is spiritual, existing within the hearts of believers (Luke 17:21). It began when Jesus preached the Kingdom and will end when He returns as the King.

Stage 3: Restoration of Israel
Stage 3 will start when Jesus will come again as King to rule the world after the Great Tribulation. The end of this stage will come when the current heaven and earth is destroyed and the new heaven and earth is created.

Stage 4: Eternal existence on the new earth
Actually stage 3 and 4 is considered as the eternal kingdom from Isaiah's point of view (Isaiah 65:17-25). Stage 4 will begin will the new heaven and new earth and will not end.