Commentary of Charles Spurgeon, Adam Clarke & Matthew Henry
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.
Vs. 25-29
Whose voice then shook the earth - Namely,
at the giving of the law on Mount Sinai; and from this it seems that it
was the voice of Jesus that then shook the earth, and that it was he
who came down on the mount. 
The removing of those things that are shaken - As of things that are made - That is, subjects intended to last only for a time.
We have now received a
kingdom that cannot be moved, shall never be removed, never give way to any
new dispensation. The canon of scripture is now perfected, the Spirit of
prophecy has ceased, the mystery of God is finished, he has put his last
hand to it.
Reverence - Godly fear - Ευλαβειας·
Religious fear.
---We have boldness to enter into the holiest by the
blood of Jesus, ---but let that boldness be ever tempered with modesty and
religious fear; for we should never forget that we have sinned, and that
God is a consuming fire.
For our God is a consuming fire - The apostle quotes Deuteronomy 4:24, and by doing so he teaches us this great truth, that sin is abominable in God's sight.
The apostle, having thus enlarged upon the argument to
perseverance taken from the heavenly nature of the gospel church, closes
the chapter by improving the argument in a manner suitable to the weight of it.
