See that you do not refuse Him who speaks. For if they did not escape Him who refused Him who spoke on earth, much more shall we not escape if we turn away from Him who speaks from heaven, whose voice then shook the earth; but now He has promised saying, “Yet once more I shake not only the earth, but also the heaven.” “Yet once more,” indicates the removal of those things that are being shaken, as of things that are made, that which cannot be shaken remain.
Therefore, since we are receiving a kingdom which cannot be shaken, let us have grace, by which we may serve God acceptably with reverence and godly fear. For our God is a consuming fire.
At Mt Sinai God shook the earth. The consequences for unbelievers now will be far dire indeed. The judgement to be experienced and the expected terror is far in excess of that on Mt Sinai. God shook the earth from Zion. He will shake the heavens, the entire universe on the Last Day…His Second Coming (Jesus).