What McGrath offers is not a theological treatment of various heretical beliefs throughout the Church's history. Instead what he offers is an academic critique, historical and psychological purposes and drives behind heresy in the Christian tradition. This is important to note. McGrath is not trying to show how Arianism or Valintenianism is heretical, he assumes them to be heretical. He seeks to critique the movements and find out why heresy is so common and what drives them.
One of the most helpful discussions in the book is McGrath's discussion of Walter Bauer's thesis that heresy is simply orthodoxy that lost its vote. The argument is that in the early years of Christianity, there was no unified doctrine and theology. Instead Christianity was made up of diverse beliefs. Though not making this the purpose of his book, McGrath shows that this theory is false. Bauer argues that orthodoxy is inherently authoritatively that snuffed out diverting opinions.
The problem with this thesis is that it is inconsistent. To make such an argument suggests that many of the early heresies (like Pelagianism) were themselves very restrictive, authoritarian, and oppressive (if we can use the word in a postmodern sense). What Bauer accuses of orthodoxy could equally be applied to heresy not to mention the fact that neither Canon of Scripture or orthodoxy was ever voted on by an oppressive council. That is simply not true.
Another important aspect of the book is the discussion of how culture and society influences theology. McGrath looks at the several heresies highlighted in the book reflect the theology and philosophy prevalent at that time. I find this to be one of the greatest motivations of heresy and liberalism. A desire to be more like the culture or the ignorance that one is being shaped by culture and not the gospel is powerful and explains much of what leads to heresy and liberalism.
Overall, this is a good book like all of McGrath's other books. Though for the new reader there are some dangers in reading the book as it would be easy to think McGrath is saying something he isn't. For example, it sounds like McGrath argues early on that there was a diversity in Christian beliefs in the Early Church. This is only true t a certain extent. The Church established orthodoxy at the cross of Christ. That was not up for discussion (just read the New Testament), but what was being debated were other areas of life.
So anyone interested in the discussion, you may like this one. If nothing else, it is an interesting read and the discussion the author gives regarding some of the more popular heresies of the Early Church are worth your read.
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Reviews - "Christianity's Dangerous Idea" by Alister McGrath
