To understand what BtG is: -
Nimrod was the founder of Babel.
Nimrod had a wife.
Nimrod was killed.
Nimrod's wife took control of the Kingdom.
Nimrod's wife claimed that she was impregnated by Nimrod from beyond the grave.
Nimrod's wife told people that the child was Nimrod.
Nimrod's wife later married the child.
Then the languages were confused and this established religion was taken to all parts of the earth.
This is the whole basis for the original trinity teaching, Father, Mother & Son, with the Son being said to be both the Father and Son and what Christmas is really all about.
Nearly every resulting Nation had its three primary gods, consisting of a Father, Mother & a Son theme.
Nearly every resulting Nation carried this belief system and allowed it to exert great influence over its Kings and people.
Revelation 17:15 Then he said to me, “The waters which you saw, where the harlot sits, are peoples, multitudes, nations, and tongues.
Revelation 17:18 And the woman whom you saw is that great city which reigns over the kings of the earth.”
Thus lays a pattern for what BtG is.
It is a religious element that traces it's belief system to Babylon.
It is associated with Tartarus and has demonic influence and backing.
2 Peter 2:4 For if God did not spare the angels having sinned, but having cast them down to Tartarus, in chains of gloomy darkness, delivered them, being kept for judgment;
Revelation 18:2 And he cried [a]mightily with a loud voice, saying, “Babylon the great is fallen, is fallen, and has become a dwelling place of demons, a prison for every foul spirit, and a cage for every unclean and hated bird!
For example, there was Rome but there was also what came to be the holy Roman empire - a Babylonish religious element that exerted great influence over Kings.
This religious element that exerted influence over Kings of the earth did not come into existence in Rome's day, this religious element has been there from the time of Babel.
In Rome's day, BtG came to include Apostate Christianity. The proposed belief that Jesus was both the Father and the Son and that he was born on December 25th is a mark of Christianity becoming one of the Harlots.
Thus BtG is not Apostate Christianity but its encompasses it or includes it.
Apostate Christianity is just one of the many Harlots with BtG being the actual mother of the Harlots.
Revelation 17:5 "And upon her forehead was a name written, MYSTERY, BABYLON THE GREAT, THE MOTHER OF HARLOTS AND ABOMINATIONS OF THE EARTH."
In Rome's day it played it's part in influencing Kings but it has been influencing Kings from Babel right up until now.
Revelation depicts a time when the Kings of the Earth will be made to turn on this belief system.
Revelation 17:16 And the ten horns which you saw on the beast, these will hate the harlot, make her desolate and naked, eat her flesh and burn her with fire. 17 For God has put it into their hearts to fulfill His purpose, to be of one mind, and to give their kingdom to the beast, until the words of God are fulfilled. 18 And the woman whom you saw is that great city which reigns over the kings of the earth.”
This will occur when Christ intervenes.
** for a lot more detailed information see the book The Two Babylons by Alexander Hislop
Revelation 10:7 But in the days when the seventh angel is about to sound his trumpet, the mystery of God will be accomplished, just as he announced to his servants the prophets.”