This was one of the most helpful books I have ever read. I had the misfortune to be reared, in part, as a Jehovah's Witness. Whereas most Christian contemporaries celebrated the moral tales and miracles of Jesus, and St. Paul's evangelism, JWs were obsessed with the imminent End of the World. Part of their prophetic scenario involved the chaining of Satan by St. Michael, and his 1000 year banishment, as related in the Book of Revelation. This banishment is the Millennium to which the title refers.
It was quite salutary to discover, in Cohn's book, that apocalyptic obsessions had, for centuries, been central to Christian belief, and not merely the province of whacko fringe movements. This is hardly surprising, given the content of so much of the Bible. The Crusades, and all their bloodshed, we read, were an attempt to fulfil Biblical prophecy.
Cohn describes a whole range of desperate and credulous people, across several centuries, who were persuaded to follow a range of Christian Revolutionaries, who railed against private property and privilege and claimed thereby to be ushering in a New age of Christian purity and Piety.
There are legends also, of the return of the King, not always Jesus, sometimes the Emperor Frederick II. 'Respectable' clerics, like St. Francis of Assisi and Joachim of Fiore, are also highlighted as stirring up apocalyptic fervour.
Some of the prophets, with their ravings against Jews and private property, foreshadow those two evils of the 20th century, Fascism and Communism. Indeed, it was the righteous certainty of members of both those movements that were the book's inspiration. As an Intelligence Officer in the British Army in WW2 Cohn came face to face with true believers in both those destructive movements. 50 years on this book has not dated. Although Islamic Fundamentalism is beyond this book's scope, it is easy to see the parallels with its mediaeval Christian equivalent.