Jesus, when He returns, will gather the nations.  These are those who have not been immediately destroyed in His then present wrath that is now taking place as He comes.  Remember, when He appears in the clouds following what will have been, for this world a time of evil rule and turmoil such as never has been and never again will happen for mortal mankind; then there will be vengeance against evil. 

His appearing will cause evil kings and leaders to react in great fear, men who have destroyed believers, the Sheep whom God the Father sent His Son to redeem off of this earth and to follow Him and bear witness of Him while here.  These leaders are at this future time shuddering in fear of the Lamb:

Revelation 6:14  The sky was split apart like a scroll when it is rolled up, and every mountain and island were moved out of their places.  15  Then the kings of the earth and the great men and the commanders and the rich and the strong and every slave and free man hid themselves in the caves and among the rocks of the mountains;  16  and they said to the mountains and to the rocks, “Fall on us and hide us from the presence of Him who sits on the throne, and from the wrath of the Lamb;  17  for the great day of their wrath has come, and who is able to stand?” (NASB1995)

When the Lamb of God appears and the earth quakes and the mountains explode in upon themselves; it will be even as the quaking in Jerusalem on the day of His death, but it will be a worldwide catastrophic event, as Jesus returns to Rapture His brethren, whom He is soon to wed.  And it is then that all in the heavens and in the earth will see Him:

Revelation 1:7  BEHOLD, HE IS COMING WITH THE CLOUDS, and every eye will see Him, even those who pierced Him; and all the tribes of the earth will mourn over Him. So it is to be. Amen.  8  “I am the Alpha and the Omega,” says the Lord God, “who is and who was and who is to come, the Almighty.” (NASB1995)

Kings and leaders men who have knowingly, willfully and gleefully gathered armies to Armageddon, who have ruled doing to His brethren (to His sheep) every imaginable evil, they will know that their rule over earth is done.  They will flee to hide from this One appearing on the clouds of the sky with His armies. The darkness is now gone and the light of the world which they hate (for they loved the darkness) He has appeared in both glory and wrath to expunge the earth of its evil.  

And when He comes the evil on the earth will be fully ripe:

Revelation 19 tells us that when He comes that He is clothed in a robe dipped in Blood, and He is accompanied with a heavenly army. Have you put on your armor. Are you in His army? 

Upon His return we his brethren will always be with Him, and He will gather the nations, with the sheep on His right and the goats on His left. And concerning the drink of water He will say:  for I was thirsty and you (did or you did not) give me a drink. 

IT IS THE ACTION CONCERNING THE DRINK OF WATER THAT WILL REVEAL THE HEART OF MAN. 

Listen: For before man the cup could  condemn one, but to God it will make one worthy:

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And so, man is revealing his heart even in these days before the coming of the Lamb.  And it is not, in just the giving of water, but it is in man’s reaction to the giving.  For the heart is known by God, and it is being revealed in these days. Its desires are evident and ever evil, or if not they are restrained by God’s Holy Spirit within you.  He is your blessed hope in the midst of the evils evident upon and soon to be  permeating this earth. If you do not yet know Him you also can find Him.