“See that ye refuse not Him that speaketh. For if they escaped not who refused Him that spake on earth, much more shall not we escape, if we turn away from Him that speaketh from heaven: whose voice then shook the earth: but now He hath promised, saying, Yet once more I shake not the earth only, but also heaven. And this word, Yet once more, signifieth the removing of those things that are shaken, as of things that are made, that those things which cannot be shaken may remain. Wherefore we receiving a kingdom which cannot be moved, let us have grace, whereby we may serve God acceptably with reverence and godly fear: for our God is a consuming fire.” Hebrews 12:25-29 (KJB)
The One we must not refuse is Jesus Christ.
The One who spoke on earth is referring to God speaking through Moses and the prophets, and Jesus as He walked with us.
His voice is so powerful that with it He shakes the universe (Psalm 29).