The kingdom has been in preparation since the foundation of the world................

Indeed we read "I am preparing a place for you" now this is past,present and future tense, deopending on who is reading that scripture and in the past, present or future life
of the person reading

Matthew 25:34 (New American Standard Bible)

34"Then the King will say to those on His right, 'Come, you who are blessed of My Father,
inherit the kingdom prepared for you from the foundation of the world.
But lets look at Past Kingdom

Now lets take the "Kingdom of God " into the garden of Eden shall we ?

How you ask was God King of Adam and Eve ?

Well again this was a Kingdom, He was King of their lifes....and He had dominion over them,
King  Dom,
After all did He not make them and form them and give them life ? why could He not then be
King ? and if He was King this was a "Kingdom" more so than an earhtly Kingdom, much more

Since when did an earthly King ever give life to man ?

Now being King God gave Adam and Eve dominion over the earth for it was His to give and He
placed authority in their hands, they became a chosen couple if you like, a Royal preisthood,
He gave them authority over the earth

He gave them the "Authority Robe" at this time and they were Holy, Adam and Eve were Holy
as they lived and breathed and talked and walked with God in the Garden in His Kingship (at this time)

Now lets jump a bit here through a few thousand years or so from what we have seen to be
the "Past tense" of Kingdom

Now in Exodus 19:6 it says
"you will be for me a kingdom of priests and a holy nation.'
This  relates to the Israelites and this is God speaking of wehat will be in the future

And we know that Israel , Gods Chsoen people were a Kingdom and at one time they had the
"I AM" as King, 1 Chronicles 29:23 as such this was a true Kingdom, not ruled by men but by
God Himself

I will not cease from mental fight,
Nor shall my sword sleep in my hand,
Till we have built Jerusalem
In England's green and pleasant land.

The Kingdom of man has become the Kingdom of our God Revelation 11:15