You use the term Kingdom wildly.
Usually “The Kingdom” refers to the Kingdom Of God.
The earthly kingdom is a 1000 year reign from the throne of David (Jerusalem).
There is zero evidence in the word that even one single soul will be saved during that reign. It is a demonstration, not a plan for salvation.
One must be the beneficiary of grace to be “changed” to a spiritual body that can exist in the realm of God.
Your definition of “the gospel of the grace of God” is not from scripture, it is man made.
The belief that saves is the belief that leads one to follow Yeshua in every way, not intellectual belief that he died for our sins. As Paul noted, Satan and his angels so believe.
I pray God’s wonderful grace and His tender mercies for you. And His truth. Always His truth.