"The Mistery of Iniquity" was a critique of the failings of Catholicism, state-church Protestantism and Brownism. It was the first ever English book to defend the principle of religious liberty. For Helwys, religious liberty was a right for everyone, whether Protestants, Catholics, Jews, Muslims or atheists. The book also argued that Separatist leaders such as Smyth and Robinson had been wrong to take their churches overseas to escape persecution and that they should return to England. The historian of Separatism Stephen Tomkins described "The Mistery of Iniquity" as the most radical and outspoken book of the age and 'the most far-reaching declaration of universal religious freedom yet seen in English', but added: 'It is a pity that this most ground-breaking treatise of the Separatist movement should also be its most mean-spirited.'