What does the word 'mystery' mean in Revelation 17:5?

Les explains that in Revelation 17:5, the word 'mystery' is the term from which we get the word 'secret.' The verse states that upon the woman's forehead was a name written, 'mystery, Babylon the Great, the mother of harlots, and the abominations of the earth.' This mystery is a hidden, symbolic truth about the great religious system of Babylon. In the preceding verses, the woman is arrayed in purple and scarlet, decked with gold, precious stones, and pearls, holding a golden cup full of abominations and the filthiness of her immoralities. Les points out that this speaks of tremendous wealth in the hands of the hierarchy while the subjects are poor as church mice. He says all the religions of the world—Shindos of Japan, Hindus, Mohammedans, Catholics, Protestants—will come under this one great umbrella. It is satanic and demonic, an amalgamation of idolatrous worship. Les clarifies that this is not so much physical sexual immorality but spiritual immorality, the amalgamating of idolatrous worship with so-called biblical worship. Thus the word 'mystery' means 'secret,' pointing to this hidden, symbolic reality of Babylon the Great.