I love a good book about heresy. All creative thought in religion comes out of heresy and every new religious movement starts out as a heresy from an older religion. For that matter pretty much all orthodox theology is written in reaction to heresy. Of course having a new religious idea is not enough to create a heresy. Someone else has to be so certain that they alone possess absolute truth that they believe they are justified in using force to compel anyone who disagrees with them to recant.
Christianity seems to be particularly fertile when it comes to spawning new heresies. After two thousand years of theological pontificating there are still core concepts that are tantalizingly vague. Every attempt to precisely explain the relationship between God the Father and God the Son inevitably leads to the explainer being accused of heresy. The process by which believers acquire salvation is also still up for heated debate.
There is a temptation to depict the heretics of old a heroes and the orthodox as villains. While it is honorable to sympathize with those who were tortured or executed for refusing to give up their unorthodox beliefs, it is important to remember that few if any were champions of religious pluralism or tolerance. The heretics were often just as convinced of their absolute correctness and when they occasionally came into power were quick to use the same tools of persecution against the ones who persecuted them.
Wright is less interested in the details of the various heresies of the past than with the great historical trends that produced them. Why did some heresies fail while others went on to become their own independent orthodoxies? Why were some time periods rife with heresy while others were relatively quiet? The greatest mystery of the history of religion may not be why people were willing to kill each other over minor points of doctrine, but why they have for the most part stopped.