[TheRealThing]

^^ "Then shall all the tribes of earth mourn." In Romans chapter one God says He has revealed himself to every man, therefore they are without excuse. Which of course means that as He said, judgment is certain.
Romans chapter 1
18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of people who suppress the truth in unrighteousness, 

19 because that which is known about God is evident within them; for God made it evident to them. 

20 For since the creation of the world His invisible attributes, that is, His eternal power and divine nature, have been clearly perceived, being understood by what has been made, so that they are without excuse.

The tribes of earth means all men of every race color and creed, will be terrified at His coming. The reason they will be terrified is because despite any protestations to the contrary, God has revealed Himself to everybody. There are supposedly genuine atheists in abundance out there who do not believe in God. A claim that in view of Romans 1:18-20, I sincerely doubt.

Revelation says at one point during the Great Tribulation 1/3 of all men will perish. At another point 1/4 of all men will perish. So millions of people will have died, likely within a 3 1/2 year time span. But the point is there will still be a lot of people alive when the Lord comes back to earth and for those who've steadfastly denied Him it will be too late. The terror and the dismay will defy our ability to describe.