‘Whose voice then shook the earth. But now he has promised, saying, “Yet once more will I make to tremble, not the earth only, but also the heaven.” '
God has spoken and will yet speak again even more terribly. For at Sinai His voice shook the earth (Exodus 19:18), and it trembled before Him. That was terrible for those who experienced it. But now His promise is that He will once again shake the earth, and not only the earth but the heaven also will tremble before Him (see Haggai 2:6). One day God was to reveal Himself as He never has before.
The ‘shaking' was to some extent already present in Jesus and the Kingly Rule of God. All was shaken when Jesus came to earth proclaiming that the Kingly Rule of God was present in Him (Matthew 10:34); and in His ministry against evil spirits and in His victory over them at the cross the heavens were shaken too (Luke 10:18). And this shaking continued in the ministry of the early church (Acts 4:31; Acts 16:26), and in the destruction of Jerusalem. More was going on than we will ever know (compare Daniel 10:11-21). It will be made even more apparent when God brings things to their final consummation (Matthew 24:29; Luke 21:26).
‘Heaven' here probably refers to the heaven made at creation, and to the sphere in which operate the spirits of evil, not to the Heaven of heavens. That has just been seen in peace and harmony as being untouchable. It is the present creation that is to be devastatingly removed (2 Peter 3:10-12). In the light of these pending events they should take the more earnest heed.