Hello,
I think that scripture doesn't support the notion that Christendom is attacked in the beginning of tribulation.
Please note that in Rev. 17:16 there is no timing given as to the beginning of the GT.
If Christendom (aka Babylon the great) falls in the beginning of tribulation, how is it then that harlot sits on the back of the revived beast that comes out of the abyss? According to Revelation 17 the seven-headed beast goes off into destruction and then comes back out of the abyss as the 8th king which is when the harlot is depicted as riding the revived beast and immediately before God puts it into the mind of the beast to devour her. Revelation in the 11th, 13th and 17th chapters talk about a beast that goes into a deathlike state (aka receives a sword stroke) only to later be revived.
Rather than the destruction of organized religion marking the commencement of the great tribulation, the tribulation apparently brings about the collapse of the present Anglo-American dominated democratic system. That is in keeping with Daniel’s prophecy, which foretells that a king of fierce countenance will bring mighty ones to ruin, as well as the sons of the kingdom. Afterwards, with the democratic nations brought low, the United Nations will be empowered to the extent of becoming a global empire (aka the 8th king).