Because it took me so long I've not got a good grasp of the whole thing in my head any more but I am still attempting to write a summary of sorts (and I really wish I'd written up notes after each section, I have a plan to do that with future books, writing about what I've read helps solidify it in my head). It opens with a couple of chapters that set the scene - a brief history of the Greeks & their philosophy (and their conquest by & absorption into the Roman Republic), followed by a brief history of Israel & the Jewish religion & philosophy. The next chapter deals with what can be teased out of the Gospels about the historical person of Jesus, and the immediate aftermath of his crucifixion - in terms of what happened in the very early Church, which at the time was really a branch of Judaism. The separation of Christianity from its Jewish roots comes with Paul (who isn't one of the original disciples), and with the fall of Jerusalem in 70AD which disrupted the still Jewish leaning part of Christianity.
Historical accident or circumstance plays a large role in the early development of the Church, in contrast to later on when the Church has a hand in creating the political history (in Europe, anyway). At first the centre of gravity of the Church was towards the east of the Mediterranean - Egypt, Syria, and even further east to Baghdad & beyond were important centres of Christianity. The westward shift comes from the intertwining of the Church in Rome with the Roman Empire, and then the later rise of Islam in the Middle East which marginalises those Christian communities. Latin & Orthodox Christianity become dominant not because they're closer to the original (they really aren't) but because they get political power and hook themselves into the state apparatus of the Roman Empire & its successors.
A common theme through the whole history of the Church is schisms based on arguments about what the stories in the Bible mean, and what they should teach us to believe & how they should teach us to behave. This starts right at the beginning with Paul and his not-necessarily-Jewish form of Christianity (that ended up completely dominating) and James the brother of Jesus, whose Christianity was a flavour of Judaism. There are splits over the nature of Jesus (both divine and human, but is that mixed or separate within the body of Christ?), splits over the concept of the Trinity, splits over whether icons are permitted or prohibited, splits over the meaning of the Eucharist (literally magical or metaphor), splits over who is in charge (Pope vs monarchs, priests vs elders), splits over the construction of the afterlife (purgatory? or not?), splits over free will or predestination. And more. It seems a bit like every possible stand one can take about the nature of God and of the message that Jesus was preaching, someone has taken and has proclaimed to be a revelation directly from God.
Another strand running throughout the history of Christianity is the imminent arrival of the end of the world. Jesus and his early followers were expecting the End to begin during their own lifetimes - his message was an apocalyptic one. One of the re-adjustments necessary in the mainstream Church after this was a move away from a literal interpretation of the end as nigh to a more metaphorical one where it is "soon" in the time of God not necessarily in human terms. But the idea keeps returning, time and time again various sects and parts of the Church have begun to believe that the End will come in their lifetimes, and time and time again it hasn't. It has survived even into modern times, affecting the political landscape - there is apparently a significant strand of support for the Israeli state that comes from a belief that a Jewish homeland is a necessary pre-requisite for the second coming of Christ. (As an aside, I find it a bit mind-boggling that people are out there trying to encourage the right conditions for the end of the world, I guess if you "know" you're going to go straight to heaven in the Rapture maybe you have a different perspective, but I just don't really understand the point of view that thinks "hey, everyone & everything will die if we just get things organised like this, let's go do it!".)
The book ends on an optimistic note for the future of the Church - the secularisation of Europe gives an impression that numbers of Christians are dwindling and it's dying out, but globally speaking this is not the case. Two of the growth areas that he talks about are South Korea and Africa, the latter being particularly surprising because you might've thought that Christianity would be linked to colonialism and thus not popular with the African peoples. But instead they have developed their own forms of Christianity, derived from but different to the Western Christianities they've grown out of.