Belloc describes how these movements began, how they spread, and how they have continued to influence the world. He accurately predicts the re- emergence of militant Islam and its violent aggression against Western civilization.

When we hear the word "heresies", we tend to think of distant centuries filled with religious quarrels that seemed important at the time but are no longer relevant. Belloc shows that the heresies of olden times are still with us, sometimes under different names and guises, and that they still shape our world.