Made it to the end of the second section (covering 250-450 AD) and found quite a bit of interesting stuff in the second half.
The general sense I got from the period was that in the third century you had multitudes of competing sects, with the Orthodox sect becoming stronger and stronger, to the point that it started looking like a good candidate for an imperial religion and association with the Roman state. Couple that with extreme religious sects being a common enemy of orthodoxy and the Roman state, and you had a good match politically.
Then in and around the end of the fourth century as the orthodox and other sects of Christianity solidified you started to see more debate over the actual theology and philosophy of the religion. One of the main points of contention around that time was over the divine/human status of Jesus. Different sects and thinkers around the time looked at the problem differently, but in the Orthodox strain there was eventually a decree over the problem. Quoting from Wikipedia:
Eventually, by the Ecumenical Council of Chalcedon in 451, the Hypostatic union was decreed—the proposition that Christ has one human nature and one divine nature, united with neither confusion nor division—making this part of the creed of orthodox Christianity
And this caused one of the first major schisms between 'Eastern' and 'Western' Christianity at the time. Around this time Christianity also started to over-take and under-develop paganism as a major force.
In and around the fifth century the orthodox strain began re-orienting itself to start winning over the masses. They made their services more elaborate, buildings more grandiose, and so on. At the time the main aim of this was to underplay the Eastern 'Arian' form. Around this time judaism started to be looked at with more suspicion as well.
The church also started developing ideas on sex and marriage around the beginning of the 5th century, abstinence being one of the major ideas coming out of it. Lots of debate, but it's interesting that even Paul Johnson, the author of the book, notes that most of these 'rules' had nothing to do with Jesus himself, or even early Christian teachings.
Then Paul Johnson spends the last couple pages talking about Augustine, who he describes as having the most profound affect on Christianity after Paul. The sense I got out of those couple pages was that Augustine, and the general motion of Christianity at the time, was pushing toward Orthodox Christianity's complete control over society. People started to be coerced into the idea that they needed to submit to the Christian God and Church or else risk eternal damnation, and this direction supposedly dictated the brunt of the medieval era.
Note that the dates I'm using are a bit flimsy as the author jumps around a lot and it's hard to keep track of what happens when, and a lot of it is a slow evolution rather than discrete events.
Next up: Mitred Lords and Crowned Ikons (450-1054)