For anyone who actually cares about the truth, there is a fascinating debunking of Hislop published by one of his former believers:
https://www.amazon.com/Babylon-Connection-Ralph-Woodrow/dp/0916938174
Ralph Woodrow was such a devotee of Hislop’s nonsense that he originally published a book pushing it: https://en.wikipedia.org/wiki/Babylon_Mystery_Religion
When he realized the truth - that Hislop had no idea of what he was talking about - he published the second book debunking Hislop’s book and his own original book.
At this point, decades later, only uneducated twits believe Hislop.
For anyone not wanting to read the whole book, here is an article in which Woodrow disavows Hislop's book:
Did the Catholic Church Have Its Origin in Paganism? Here's one quote from Woodrow's article:
My reason for pulling the original book out of print was quite basic: Citing similarities between Catholic practices and pagan practices proves nothing if there is no actual connection. One could take virtually anything—even McDonald’s golden arches—and do the same: The Encyclopedia Americana (article: “arch”) says the use of arches was known in Babylon as early as 2020 B.C. As Babylon was called “the golden city” (Is. 14:4, KJV), can there be any doubt about the origin of the golden arches? As silly as this is, this is the type of proof that has been offered again and again about the supposed pagan origins of the Catholic Church.