Hebrews 12:24-26 KJV And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than that of Abel. 25 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: 26 Whose voice then shook the earth: but now he hath promised, saying, Yet once more I shake not the earth only, but also heaven.
Let's read that again in the ESV translation for better clarity.
Hebrews 12:24-26 ESV and to Jesus, the mediator of a new covenant, and to the sprinkled blood that speaks a better word than the blood of Abel. (25) See that you do not refuse him who is speaking. For if they did not escape when they refused him who warned them on earth, much less will we escape if we reject him who warns from heaven. (26) At that time his voice shook the earth, but now he has promised, “Yet once more I will shake not only the earth but also the heavens.”
It is true that God speaks today. He speaks by the blood that was poured out on Calvary's cross. He speaks through the blood of His saints as they die a martyr's death. He speaks by way of His angels. He spoke His words to Moses who relayed them to the people. He spoke through His Son as He walked on earth among men. But now, God mainly speaks from heaven and from His written Word.
When God speaks to mankind, especially those who are His children, He expects from them the most strict attention. God, the Father and Creator now speaks from a higher and more glorious seat and throne, not from mount Sinai, which was on this earth, but from heaven. Our heavenly Father also speaks immediately and daily through His inspired Word and by the Person of the Holy Spirit, both of which are His witnesses. He does not speak any new thing to men, but by His Spirit He speaks the same Word, because does not change. His voice once shook the earth, but now, He is shaking the heavens.
When God speaks to men the guilt of those who refuse Him is greater and their punishment will be more unavoidable and intolerable because of the two witnesses of His Word and His Spirit. There will be no escaping it or no bearing it. We have a received a kingdom that cannot be moved, shall never be removed, never give way to any new man-made kingdom.