A.T. Robertson 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)

Then shook (εσαλευσεν τοτε). Old verb as in Mt 11:7.

He hath promised (επηγγελτα). Perfect middle indicative of επαγγελλω and it still holds. He quotes Hag 2:6.

Will I make to tremble (σεισω). Old and strong verb (here future active) σειω, to agitate, to cause to tremble as in Mt 21:10. The author applies this "yet once more" (ετ απαξ) and the reference to heaven (τον ουρανον) to the second and final "shaking" at the Second Coming of Jesus Christ for judgement (9:28).