Revelation 18

 John said the angel declared as - 

"...he cried mightily with a strong voice, saying, Babylon the great is fallen, is fallen, and is become the habitation of devils, and the hold of every foul spirit, and a cage of every unclean and hateful bird.

3 For all nations have drunk of the wine of the wrath of her fornication, and the kings of the earth have committed fornication with her, and the merchants of the earth are waxed rich through the abundance of her delicacies.

4 And I heard another voice from heaven, saying, Come out of her, my people, that ye be not partakers of her sins, and that ye receive not of her plagues.

5 For her sins have reached unto heaven, and God hath remembered her iniquities."  (Revelation 18:2–5)

It appears to me that this will take place before and during His second coming.  The warning to the saints is to "come out of her".