In Hebrews 12, the writer spends a good deal of time contrasting the New Jerusalem, symbolic of the kingdom, with the fleshly kingdom of Israel which was founded at Sinai. Using imagery taken from Moses and Haggai, he outlines this contrast.
Heb 12:25-26: “Watch out that you do not refuse the one who is speaking! For if those did not escape when they refused the one who warned them on earth, much less will we escape, if we reject the one who warns from heaven, whose voice shook the earth at that time, but now he has promised, saying, “Yet once more I will shake not only the earth but also heaven.””
Hag 2:6-7,9: “For thus says Yahweh of hosts: ‘Once again, in a little while, I will shake the heavens and the earth and the sea and dry land. I will shake all the nations so that the treasure of all the nations will come, and I will fill this house with glory,’ says Yahweh of hosts. … ‘The latter glory of this house will be greater than the former,’ says Yahweh of hosts, ‘and in this place I will give peace’ declares Yahweh of hosts.’””
Heb 12:27-28: “Now the phrase “yet once more ” indicates the removal of what is shaken, namely, things that have been created, in order that the things that are not shaken may remain. Therefore, since we are receiving an unshakable kingdom, let us be thankful, through which let us serve God acceptably, with awe and reverence.”
The fleshly kingdom of Israel was of this creation and therefore susceptible to shaking and removal. However, this kingdom is unshakeable because it is not of this world.
John 18:36: “Jesus replied, “My kingdom is not of this world. If my kingdom were of this world, my servants would be fighting so that I would not be handed over to the Jews. But now my kingdom is not from here.””
Though its foundations are beyond this creation, its influence is exerted in the lives of those here who are submitted to God and his eternal throne.