Apostol § 333

Brethren, See that you do not refuse Him that speaks. For if they did not escape 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 heaven...” And this word, “Yet once more,” indicates the removal of those things that are shaken, as of things that are made, that those things which cannot be shaken may remain.