SEE TO IT THAT YOU DO NOT REFUSE HIM WHO IS SPEAKING: Blepete (2PPAM) me paraitesesthe (2PAMS) ton lalounta (PAPMSA):

See to it  (991)(blepo) in Greek writings from Aeschylus down means to see or discern. In contrast to being blind it means to be able to see. Figuratively it speaks of spiritual perception. It means to to take in the sight of something and so to look at or observe. To process information by giving thought or directing one’s attention to something. As used in this verse it means to pay especially close attention to something, especially regarding something that is needed or is hazardous (watch, beware of).

Refuse (3868)  (paraiteomai from pará = aside and in this word gives a nuance of aversion or repudiation + aitéo = ask, beg) is literally to ask along side. To seek to turn aside by asking. As in Mark 15:6, this verb can mean to beg or request (a prisoner to be freed on the occasion of the Passover). In Luke 14:18, it conveys the sense of to beg off or of wanting to be excused from a positive response, in this verse one excusing himself for not accepting a wedding invitation. Finally, in the pastoral epistles (1,2 Timothy, Titus - see below), the meaning is to decline, refuse,  to refuse to pay attention to, to shun, to avoid, to reject. In secular Greek a wrestler was declared the victor when his opponents declined to engage him upon seeing his unclothed physique.

The writer has already insisted that it was God’s voice that Israel subsequently chose to disregard in the wilderness (see note Hebrews 3:7;  3:8; 3:9; 3:10; 3:11, Hebrews 3:16;  3:17; 3:18). But in a sense Moses was the mediator when he spoke and since he spoke for God as His prophet the distinction is somewhat inconsequential.

This warning takes us back to the first warning of (see notes Hebrews 2:1; 2:2; 2:3). In Hebrews 2:1 the Hebrews were in danger of drifting away from that which they had heard; Hebrews 12:25 they also stand in peril of refusing him who speaks. Hebrews 2:2 reminded that violations of the law received immediate punishment; Hebrews 12:25 they are also told that those who refused the One who gave commandments from the mountain did not escape. There the question confronted them: Hebrews 2:3 “How shall we escape if we neglect so great a salvation?” Hebrews 12:25 the question of the first warning is now made a declarative statement: We are even less likely to escape, if we turn away from God Who warns us from heaven.

What folly! His word is effectual, and it is final.

Jesus is speaking (see notes Hebrews 1:2, Hebrews 2:1; 2:2; 2:3, cp Hebrews 3:7, Hebrews 3:16). Those who disobeyed the voice of God as it was heard in the law were punished accordingly. Hebrews 2:2 When privilege is greater, responsibility is also greater. In Christ, God has given His best and final revelation. Those who reject His voice as it now speaks from heaven in the gospel are more responsible than those who broke the law. Escape is impossible.

FOR IF THOSE DID NOT ESCAPE WHEN THEY REFUSED HIM WHO WARNED THEM ON EARTH: ei gar ekeinoi ouk exephugon (3PAAI) epi ges paraitesamenoi (AMPMPN) ton chrematizonta (PAPMSA):

Escape (1628)(pheugo) means to move quickly from a point or area in order to avoid presumed danger or difficulty. To flee, to run or move hastily from danger because of fear.

Warned (5537) (chrematizo from chrema = an affair, business) means to impart a divine message or make known a divine warning by giving a message.

MUCH LESS SHALL WE ESCAPE WHO TURN AWAY FROM HIM WHO WARNS FROM HEAVEN: polu mallon hemeis oi ton ap ouranon apostrephomenoi: (PMPMPN) polu mallon hemeis oi ton ap ouranon apostrephomenoi: (PMPMPN):

Turn away (654)(apostrepho from apo = away from, a marker of dissociation, implying a rupture from a former association and indicates separation, departure, cessation, reversal + strepho = turn quite around, twist, reverse, turn oneself about) in the active sense means cause one to change one's belief, to mislead from proper belief or to cause someone to turn away from a previous belief. To cause to turn away (apo). To turn away from allegiance. As used in this verse In the passive sense or reflexively, it means to turn oneself away from. E.g., Paul used apostrepho in this way earlier to remind Timothy

AND HIS VOICE SHOOK THE EARTH THEN: ou e phone ten gen esaleusen (3SAAI) tote:

Moses records...

Now Mount Sinai was all in smoke because the LORD descended upon it in fire; and its smoke ascended like the smoke of a furnace, and the whole mountain quaked violently. (Exodus 19:18)

The initial historical event where God’s voice shook the earth was at Mt Sinai when He spelled out the Ten Commandments with a thunderous voice. Imagine how terrifying it was to have the ground under one’s feet tremble in response to God’s audible word. There were no sleepers in the congregation at Sinai!

BUT NOW HE HAS PROMISED SAYING, "YET ONCE MORE I WILL SHAKE NOT ONLY THE EARTH, BUT ALSO THE HEAVEN: nun de epeggeltai (3SRMI) legon: eti apax ego seiso (1SFAI) ou monon ten gen alla kai ton ouranon:

Promise (1861) (epaggello from epí = intensifies meaning + aggéllo = tell, declare) means to announce with certainty as to what one will do. In Classic Greek it was used of announcing a summons or issuing a command. The perfect tense indicates promise still valid.

Shake  (4579) (seio, English seismic) means to rock or vibrate sideways or to and fro, to agitate (in any direction) to cause to tremble.   

OT prophets allude to this shaking....

For thus says the LORD of hosts, 'Once more in a little while, I am going to shake the heavens and the earth, the sea also and the dry land. (Hag 2:6) (Comment: The scope of these words points to a yet future fulfillment at the time of the second coming of Christ. Note that At Mt. Sinai God’s voice caused an earthquake, but when He speaks in the future His voice will also produce a "heaven-quake"! )

And the LORD roars from Zion And utters His voice from Jerusalem, And the heavens and the earth tremble. But the LORD is a refuge for His people And a stronghold to the sons of Israel (Joel 3:16)  (Comment:  The Lord is "the Lion of the tribe of Judah" [see note Revelation 5:5], and His powerful coming will be to the nations like the triumphant roar of a mighty lion, shaking the very heavens.)

The first shaking of the earth which is mentioned was at Mt Sinai in connection with the giving of the Law as noted above (Exodus 19:18) Another shaking, which will be attended by a total physical upheaval, will be at the return of Christ. 

Compare the prophecy in this verse to the writer's earlier declaration...

And, "THOU, LORD, IN THE BEGINNING DIDST LAY THE FOUNDATION OF THE EARTH, AND THE HEAVENS ARE THE WORKS OF THY HANDS;
11 THEY WILL PERISH, BUT THOU REMAINEST; AND THEY ALL WILL BECOME OLD AS A GARMENT,
12 AND AS A MANTLE THOU WILT ROLL THEM UP; AS A GARMENT THEY WILL ALSO BE CHANGED. BUT THOU ART THE SAME, AND THY YEARS WILL NOT COME TO AN END." (See note Hebrews 1:10; 1:11; 1:12)

To those who are obedient this is good news. And the writer means it to be a powerful encouragement to the beleaguered little church to which he writes, in which some feel as though their lives are being shaken to pieces. It is as if the writer is saying "Stand firm amidst the tremors, because the ultimate shaking is coming when the entire present evil order will fall into oblivion by the power of God. And you, as part of the new covenant people, will survive. So hangeth thou in them. Take heart!” On the other hand, to those who are ignoring God’s word and drifting further away, this was a disquieting revelation and a challenge to obedience.

Our Daily Bread - Earthquake Power - Have you ever been through an earthquake? Several years ago a mild quake awoke me with the swaying and trembling of the house. It was not severe and did not greatly disturb me. I am told that a really severe earthquake is a fearful experience. Much of the fear, however, may depend on the view one takes of the phenomenon.

During an earthquake that occurred many years ago, the inhabitants of a village were extremely alarmed. Yet they were also surprised at the calmness exhibited by an old woman whom they all knew. Eventually one of them asked the woman, "Aren't you afraid?" "No," she answered. "I rejoice to know that I have a God who can shake the world!" She had no fear because of her confidence in her God, who could rattle the world in His hand.

There is a future "shaking," a final universal earthquake coming. In Hebrews 12 we read, "Yet once more I shake not only the earth, but also heaven" (v.26). So great will this cataclysm be that Isaiah tells us "the earth will move out of her place" (Isa. 13:13). In that day we'll be safe with our Lord, and we'll be glad that He who shakes the universe is our God and our Savior. —M.R. De Haan

Great God of wonders! All Thy ways
Are matchless, Godlike, and divine;
But the fair glories of Thy grace
More Godlike and unrivaled shine. --Davies

Nothing can shake those who are secure in God's hands.