Sometimes the demons are forced to speak the truth. Tom Horn gives his version of why the damsel in Acts 16:16 said the truth about St Paul.
Traditionally we Catholics have believed that just as the Jews were prepared for the real Messiah by for instance the Blessing of Judah by his father Jacob or Isaiah or Micha, so also the Pagans were prepared, and the Cumean Sibyl was part of it.
The very verses "freemers" use on the one dollar bill and the great seal of the united states about what they want as a New World Order, Catholics have interpreted about the New World Order that began in Nazareth, Bethlehem and Calvary.
You see, Apocalypse 20 makes better sense if the end is a resumé of chapter 19 than if it all comes after chapter 19.*
And that means that the Millenarian reign has been ongoing for two thousand years now. The first resurrection is the resurrection of souls from the powers of sin, and those that reign with Christ through the thousand years include St Paul and St Peter and the Prison Guard whom St Paul did not want to get into trouble, but who washed his stripes, later same chapter.
And that means the Cumean Prophecy just as the Old Testament prophecies were already fulfilled. That is why Cumean Sibyl is there.
Virgil is very strong on the Messianic theme. He was a Pagan, and so he probably erroneously believed himself that Caesar Augustus was a Messianic man of destiny and even a god. But we who know better think the Sibyl honoured there spoke of the true God and the true Messiah, born under the reign of Caesar Augustus.
And the new breed she speaks of, is the Church. Being Pagan, she put it badly, but not badly enough to make that unintelligible.
However, that does not mean the Sibyl is not dangerous now, just as - you mentioned yourself - Old Testament is dangerous via Jews looking for the wrong Messiah.
Saturn is more than one thing. When Romans said that Carthaginians worshipped Saturn, they meant Moloch, thus Satan. When Roman poet Ovid recalls Saturn's rebellion against his Father Coelus (Ouranos in Greek) that also means Satan. But "the reign of Saturn" is something else, insofar as it is the "Golden Age", i e Paradise. And there never was a true nor ever will be a true reign of Satan all over mankind. In Nimrod's days there were Hebrews who refused to build at the tower. In Antichrist's days there will be Christians refusing to bow down to him (rapture described in Thessalonians [Second Thess.?] is by the way, as Rob Skiba correctly noted, post-Tribulation, not pre- and not mid- but post-tribulation.) So looking back at reign of Saturn as reign of Satan does not make total sense. If it was in a way referring to say the reign of Nimrod, there had been mixed with it so much falsehood about that time which was so true about the time Adama and Eve had in the garden, that it was at least as much referring to Eden.
And on the Cross, Paradise was regained.
Now, as to Obelisk in Vatican, I saw another video which reveals its Christian meaning.
Its shadows over solstices and equinoxes form an IOTA and a XI. IOTA for IHCOYC and XI for XPICTOC - meaning a confession that IHCOYC from Nazareth is the real and only XPICTOC. And, as mentioned by TomHorn, though belittled, Pope Sixtus conducted an exorcism on the obelisk before or while raising it there.
I do not think that George Washington or his guys conducted any exorcism on the obelisk in DC, though.
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Feast of St Alphonsus Maria Liguori
*Confer St Augustine, who does not totally reject all more literalist readings of chapter 19 and 20 but the carnal ones, but prefers reading first resurrection as that of the soul: Book 20 of City of God. Chapters 8-10 or start even before 8.