This book, initially published in 1853 as a pamphlet, was greatly revised [citation needed] and expanded and released as a book in 1858.
In the book, Hislop argued that the Catholic Church was a Babylonian mystery cult and pagan, but Protestants worshipped the true Jesus and the true God. He believed that Catholic religious practices are pagan practices that were grafted onto true Christianity during the reign of Constantine. Then, the merger between the Roman state religion and its adoration of the mother and child was transferred to Christianity, merging Christian characters with pagan mythology. The goddess was renamed Mary, and Jesus was the renamed Jupiter-Puer, or "Jupiter the Boy".
Hislop believed that the goddess, in Rome called Venus or Fortuna, was the Roman name of the more ancient Babylonian cult of Ishtar, whose origins began with a blonde-haired and blue-eyed woman named Semiramis.[citation needed]
According to Hislop, Semiramis was an exceedingly beautiful woman, who gave birth to a son named Tammuz, was instrumental as the queen, and wife of Nimrod the founder of Babylon, and its religion, complete with a pseudo-Virgin Birth. Later, Nimrod was killed, and Semiramis, pregnant with his child, claimed the child was Nimrod reborn. [citation needed]
Hislop attempted to show that the cult and worship of Semiramis spread globally, her name changing with the culture. In Egypt she was Isis, in Greece and Rome she was called Venus, Diana, Athena, and a host of other names, but was always prayed to and central to the faith which was based on Babylonian mystery religion. According to Hislop, Constantine, though claiming to convert to Christianity, remained pagan but renamed the gods and goddesses with Christian names to merge the two faiths for his political advantage.[citation needed]
Constantine's endorsement of the tradition was a turning point for Early Christianity. In 313, Constantine issued the Edict of Milan legalizing Christian worship. The emperor became a great patron of the Church,[citation needed] and set a precedent for the position of the Christian Emperor within the Church and the notion of orthodoxy, Christendom, and ecumenical councils that would be followed for centuries as the State church of the Roman Empire.