Whose voice then shook the earth - When he spake at Mount Sinai. The meaning is, that the mountain and the region around quaked; Exodus 19:18. The "voice" here referred to is that of God speaking from the holy mount.
But now hath he promised, saying - The words here quoted are taken from Haggai 2:6, where they refer to the changes which 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 advent 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 wilt 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 lays emphasis on the fact 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 either:
(1) to the extraordinary phenomena in the heavens at the birth, the death, and the ascension of Christ; or.
(2) to the revolutions in morals and religion which 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;" or.
(3) it may be more literally taken as denoting that there was a remarkable agitation in the heavens - in the bosoms of its inhabitants - arising from a fact so wonderful as that the Son of God should descend to earth, suffer, and die.
I see no reason to doubt that the latter idea may have been included here; and 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 Mount, the introduction of the gospel agitated the universe. Great changes upon the earth were to precede it; one revolution was to succeed another preparatory to it, and the whole universe would be moved at an event so extraordinary. The meaning is, that the introduction of the gospel was a much more solemn and momentous thing than the giving of the Law - and that, therefore, it was much more fearful and dangerous to apostatize from it.
THE REVOLUTION HAS BEGUN!