God’s voice shook the earth at that time, but now He has promised, saying [Hag. 2:6], “I will cause not only the earth to shake once more, but also heaven [as well].”
Now the words, “once more” indicate the removing of what can be shaken --- that is, the created universe --- so that those things which cannot be shaken will remain. [Note: These “things” have been variously interpreted to be eternal truth, the heavenly kingdom, the new heavens and earth, etc.].