"See to it that you do not refuse him who speaks. If they did not escape when they refused him who warned them on earth, how much less will we, if we turn away from him who warns us from heaven?" NIV translation
This verse speaks of the fact that Jesus Christ is superior to Moses. Moses gave the Law at Sinai and those who would not listen perished. As Christ is superior to Moses, how could there be any hope for those who refuse to listen to Him? The "turn away" is speaking of the Hebrew Christians going back to trusting in their religious actions instead of the finished work of Jesus.
"At that time his voice shook the earth, but now he has promised, 'Once more I will shake not only the earth but also the heavens.' The words 'once more' indicate the removing of what can be shaken - that is, created things - so that what cannot be shaken may remain." NIV translation
The earth shook at Sinai with the Law but, in the last days, all of creation will be shaken. The shaking is a picture of separating like wheat from the chaff. In the final shaking, all that will remain is the kingdom of God.
"Therefore, since we are receiving a kingdom that cannot be shaken, let us be thankful, and so worship God acceptably with reverence and awe, for our 'God is a consuming fire.'" NIV translation
What is acceptable worship? We see that it is being thankful that an all powerful God has chosen to dwell among us. It is an awareness that God is above all things and all things are subject to Him.