In Revelation 18:4, the message is clear: Jesus is calling those He deemed to be 'My People' to leave Babylon, to come out of Babylon.

What is Babylon where we find 'My people', where 'My people' got ensnared?

The Babylon is a city, a great city (not a country or a great country), as we learn in Revelation 18:10, 21. Already, in the passages, we can understand that the world where we live is not Babylon.

"standing afar off for the fear of her torment, saying, Woe, woe, the great city, Babylon, the strong city! for in one hour is thy judgment come." Revelation 18:10 ASV
"And a strong angel took up a stone as it were a great millstone and cast it into the sea, saying, Thus with a mighty fall shall Babylon, the great city, be cast down, and shall be found no more at all." Revelation 18:21 ASV

Think about it, can 'My people' fit in one literal city (not even a country)?

Also, Babylon is not any ordinary city when we read that we find in her 'the blood of the Prophets'. 

Were the Prophets ever killed in a country called Babylon for the book of Revelation to say that Babylon was involved in shedding the blood of the Prophets? No!

As far as we are concerned about the blood of the Prophets, Israel must be Babylon because that is the country where the Prophets of Jesus (God) were all killed.

"Revelation 18:23-24 ASV
(23) ... for with thy sorcery were all the nations deceived.
(24) And in her was found the blood of prophets and of saints, and of all that have been slain upon the earth."

However, Babylon of the Apocalypse cannot relate to Israel of God. Is that possible? Meditate on that.

Inevitably, the apocalyptic Babylon identification is similar to the 'born again' extra-ordinary dilemma of Nicodemus that required a spiritual approach to understand, because the doctrine of the great city Babylon is also extra-ordinary.

According to Revelation 18:23, note that the great city Babylon uses her sorcery to deceive all the nations, every single nation!

Again, that great city Babylon cannot be our living world that is made of other countries and cities besides Babylon, countries that Babylon has the power to deceive.

Is it written anywhere that Jerusalem of Israel who shed the blood of the Prophets will become the great city Babylon that uses sorcery to deceive all the nations? Negative!

What is and where is the Apocalyptic Babylon that is hosting real 'My people' believers of Jesus, a people who must leave Babylon voluntarily on their own accord, as an act of obedience to Jesus?

Because 'My people' is real, Babylon cannot be a fictitious great city.