NOTES for Heb 12:28-29
For Paul, Christianity is above all the Kingdom and life in the Kingdom. The realities of our fallen world are by no means absolute for him. Only God's being is fully and absolutely real; being whose existence God supports directly and immediately, as He supports the world He created, can be considered relatively real. But for the world's existence to become possible, it must exist with God's presence as its center.
The created world is a theophany, a manifestation of God, which, however, is not identical with God in His essence. And the created world as such, matter, the substance of which it consists, is only the outer boundary of this theophany. This is how the world appeared before Moses and his people at Sinai: in the center was the theophany, God's presence revealed to the world, the sacred mountain, and around it the rest of the world as the periphery of the sacred mountain, as the vessel of the theophany.
But that was only a projection of the universe as it is, while with the coming of Christ, when the world stands before the throne of glory because Christ opens to the world the reality of this throne, what is revealed to humanity is no longer a projection but the universe itself in all its fullness, the universe as the Kingdom of God. Meanwhile, however, man remains man, fallen man, over whom sin has, if not absolute, then decisive power. And the author of the letter reminds his readers that one cannot live in the Kingdom, before the throne of glory, without resisting one's sinfulness.
Refusal of such resistance, or still more ignoring the reality of the Kingdom, means only one thing: the person who ignores it, refusing to live in the sphere of God's love, comes face to face with God's power. And such a collision can end in only one way: the person simply disappears, ceases to exist, as every finite quantity disappears before infinity. If God has not become Love for a person, He becomes for him a "consuming fire." This, of course, is also possible, but it is not at all what God wants for man. Yet a person can come to know God's fire if he rejects God's love.