Albert Barnes Commentary Hebrews 12:26

"whose voice then shook the earth: but now he hath promised, saying, Yet once more will I make to tremble not the earth only, but also the heaven." — Hebrews 12:26 (ASV)

Whose voice then shook the earth. This happened when He spoke at Mount Sinai. The meaning is that the mountain and the surrounding region quaked (Exodus 19:18). The voice referred to here is that of God speaking from the holy mountain.

But now He has promised, saying. The words quoted here are taken from Haggai 2:6, where they refer to the changes that would take place under the Messiah. The meaning is that there would be great revolutions in His coming, as if the universe were shaken to its center. The apostle evidently applies this passage, as it is done in Haggai, to the first coming of the Redeemer.

I shake not the earth only. This is not quoted literally from the Hebrew, but the sense is retained. In Haggai it is, Yet once, it is a little while, and I will shake the heavens, and the earth, and the sea, and the dry land; and I will shake all nations, and the desire of all nations shall come. The apostle emphasizes that not only the earth was to be shaken, but also heaven. The shaking of the earth here evidently refers to the commotions among the nations that would prepare the way for the coming of the Messiah.

But also heaven. This may refer to one of several possibilities:

1. To the extraordinary phenomena in the heavens at the birth, death, and ascension of Christ;
2. To the revolutions in morals and religion that would be caused by the introduction of the gospel, as if everything were to be changed—expressed by "a shaking of the heavens and the earth";
3. Or, it may be more literally taken as denoting that there was a remarkable agitation in the heavens—in the hearts of its inhabitants—arising from a fact so wonderful: that the Son of God would descend to earth, suffer, and die.

I see no reason to doubt that this latter idea may have been included here. The meaning of the whole, then, is that while the giving of the law at Mount Sinai—fearful and solemn as it was—was an event that merely shook the earth in the vicinity of the holy Mountain, the introduction of the gospel agitated the universe. Great changes on the earth were to precede it; one revolution was to succeed another preparatory to it, and the whole universe would be moved at such an extraordinary event. The meaning is that the introduction of the gospel was a much more solemn and momentous event than the giving of the law, and that therefore it was much more fearful and dangerous to apostatize from it.