It is important to separate sincere Catholics from the history of the Catholic Church. God does note the difference between what people believe/are taught/are associated with and their own good works -though any misconception can lead to error and lead even the sincere astray.
While I do not agree with some Catholic teachings, whenever I have been in need, Catholics have been there to assist.
The idea comes from the association of that church, as an institution, with human government and putting that relationship before purity before God.. and things such as the Inquisition, the belief that the Popes are all successors of Peter, changing the sabbath commandment to Sunday observance and declaring sabbath-keepers anathema from Christ, associating pagan holidays with the name of Christ rather than keeping God's holy days of the bible, changing other teachings, etc. -depending on who you talk to.
Christ was "officially" executed by the Roman government (though at the request of some Jews of the day), yet somehow those who claimed to have the authority of Christ gained great political power over great empires -beginning with Rome. The problem is that their teachings -though similar at first glance -are very different than what is written in the bible -and they targeted those who kept the commandments of God -especially the sabbath.
The idea is that it was a false church which targeted the true church -the "little flock" -and used the popular name of Christ to gain power.
Some of the differences include the teaching that the spirit of God is a separate third person (trinity doctrine) rather than being the power by which God acts -literally his spirit which he can put within man (In the beginning was the Word, and the Word was with God, and the Word was God -that is two -not three -and the spirit of God moving upon the face of the waters decribes how things were created -not that the spirit of God was a separate third person) -the idea that people go to heaven or hell when they die -rather than the gospel of "the kingdom" where those who die are as asleep until the first resurrection -then reign with Christ on Earth as described in the bible. (The meek shall inherit the earth). Though that is only the beginning of the kingdom.
Christ, the apostles and New Testament church kept the sabbath and holy days of the bible -and those who continued to keep the sabbath would have eventually been labeled "Judaizers" -declared anathema from Christ (Council of Laodicea) -and not treated well -to say the least.
That is believed to be described here..... (Passover and the days of unleavened bread were essentially replaced by Easter. Easter happens to be another spelling of Astarte or Ishtar -chief goddess of the Babylonian mystery religion -and the associated symbols taken from fertility rites, etc.)......
Rev 17:5And upon her forehead
was a name written, MYSTERY, BABYLON THE GREAT, THE MOTHER OF HARLOTS AND ABOMINATIONS OF THE EARTH.
6And I saw the woman drunken with the blood of the saints, and with the blood of the martyrs of Jesus: and when I saw her, I wondered with great admiration
Technically, it is considered the harlot that sits astride the beast due to the interrelationship with the human governments described in scripture as the beast that rises from abyss, etc.... However, their relationship is not always pleasant -and it is written that the "beast" will actually destroy the harlot in the end times.
Here are some other associated verses....
Rev 17:9 Here is the mind which has wisdom. The seven heads are seven mountains on which the woman sits
(Some believe the above to be a reference to the seven hills of Rome)
Rev 17:
15And he said to me, "The waters which you saw where the harlot sits, are peoples and multitudes and nations and tongues.
16"And the ten horns which you saw, and the beast, these will hate the harlot and will make her desolate and naked, and will eat her flesh and will burn her up with fire.
17"For God has put it in their hearts to execute His purpose by having a common purpose, and by giving their kingdom to the beast, until the words of God will be fulfilled.…