The topic of The Kingdom of God is of fundamental importance to the NT writers. Jesus spoke more about it than on any other subject. Perhaps a good start is to study Matthew 13, where the kingdom is said to be a person in action - sowing (not reaping), planting good seed, searching, fishing.
The kingdom is dynamic activity, and who is that person doing these things? God.
The Kingdom is past, present and future, because it is the Kingdom of God, it is not the kingdom of humans. The early English meaning of "kingdom" is "kingship", or rulership, which means it is the rulership of and by God. The ekklesia is not the kingdom, but is a product of the kingdom's activity.
You will have much eye-opening enjoyment as you research this topic. I have several challenging books on the subject on my bookshelves.
Doug