Summary: Are you afraid to approach God? It could be that you should be - if you do not have Jesus - but perhaps you are afraid because you are coming to the wrong mountain - Mt Sinai instead of Mt Zion. Also learn about a city that cannot be shaken - ever.
Earthquakes are the worst kind of disaster - unlike hurricanes and tornadoes; you have little if any warning of an earthquake. Despite many years of research, scientists have yet to accurately predict where, when, and how big an earthquake will be.
Just weeks ago in Pakistan a 7.4 quake killed over 50,000 people. If you’ve ever experienced an earthquake you know the disorienting feeling it gives - it throws you off balance and you realize that if the earth beneath you moves, rolls, shakes, or breaks - then nothing at all is safe.
As bad as the earthquakes we have experienced or have heard about are - it’s nothing compared with what’s coming. The Book of the Revelation records a coming quake that will be the largest ever experienced in earth’s history. In chapter 16 we read that it will be so big that the cities of the nations will fall, islands will flee away, and mountains will come crashing down to the earth. The loss of life will be tremendous.
That earthquake will be but a foreshadowing of another kind of quake - not an earthquake only, but a quake of earth and heaven that will see the entire planet destroyed.
Science fiction writers have often thought about what would happen if the earth were destroyed or became inhabitable. They theorize about giant rocket ships transporting earth’s population to Eden-2 or something like that. As wonderful as that sounds - it’s not going to happen - at least not by man’s efforts.
In fact, the shaking of planet earth to its core is foretold in 2 Peter:
2 Peter 3:8-10 (quickview)  But do not overlook this one fact, beloved, that with the Lord one day is as a thousand years, and a thousand years as one day. 9 The Lord is not slow to fulfill his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance. 10 But the day of the Lord will come like a thief, and then the heavens will pass away with a roar, and the heavenly bodies will be burned up and dissolved, and the earth and the works that are done on it will be exposed. ESV
What will happen to you when that ultimate earthquake occurs? Imagine every molecule of every element disintegrates around you. How would you hope to survive? What you need is a safe refuge; a mountain or a city that is indestructible - one that can withstand any force - even the destruction of planet earth. Which would you put stock in - this planet that can be destroyed, or another abode that cannot? That is what the last half of Hebrews is all about.
The Israelites had been rescued from Egypt and were being introduced to their husband, and their God - Yahweh. They got a lot more than they bargained for - the result of their encounter was fear and trembling. The author of Hebrews uses this encounter to make a vital point about the differences between the Old and New Covenant - and presents a clear choice and a warning about the consequences of choosing unwisely.