Once more God will shake heaven and earth
Hebrews quotes Haggai's promise that God will once more shake heaven and earth while directing its readers toward the kingdom that cannot be shaken.
See to it that you do not refuse Him who speaks. For if the people did not escape when they refused Him who warned them on earth, how much less will we escape if we reject Him who warns us from heaven? At that time His voice shook the earth, but now He has promised, “Once more I will shake not only the earth, but heaven as well.” The words “Once more” signify the removal of what can be shaken—that is, created things—so that the unshakable may remain. Therefore, since we are receiving an unshakable kingdom, let us be filled with gratitude, and so worship God acceptably with reverence and awe.
Haggai 2:6-9 promises a shaking of heaven, earth, sea, dry land, and the nations; Hebrews 12:25-28 quotes the promise and contrasts what can be removed with the unshakable kingdom.