Basically the prophecy of doom depended on the numerology of the Revelation of St John, clarified (if that is the right word) by hints in Daniel xii, and reinforced by a blood-curdling passage in Isaiah. To those who had sufficiently studied the question there seemed to be no doubt that all history since the first year of Our Lord was divided into a series of cycles, complicated permutations of multiples of ten and seven, each cycle terminated by some gigantic event, and the whole series closing with awful finality in 1588.

Garrett Mattingly, The Defeat of the Spanish A…, loc. 396