The history of the church is the history of Christianity in relation to the world around it. MacCulloch wants to establish that the most important events in the history of the church are events which do not take place within the church such as councils and reformations, but secular political events.
I think he has a point. We Christians tend to search church history for theological purposes, and we tend to ignore what we aren’t interested in. The result can be an incomplete explanation of events which leads to a failure to understand them. Luther only succeeded because his reformation became, in the providence of God, politically viable in a way that Wyclife’s or Hus’s never did, for example. We can’t evaluate something we don’t understand. When we attempt to evaluate without understanding, we end up with junk history. History is about remembering honestly, and one of the things MacCulloch can help us to do is to remember those parts we may wish to ignore.
I do think his thesis is overstated, and even that his book does not quite bear it out. But the important thing is to remember that political events have shaped outcomes in the church, even as theological ideas have. The point is to remember accurately, not to favor one influence over the other but to understand what happened and the degree to which each influence shaped the outcome or situation we are remembering. I don’t think we can understand something we can’t explain, and we can’t explain it unless we investigate thoroughly.
I have noticed that this book’s sentences and paragraphs are repeated at times exactly in his greater work on Christianity. So this work appears to be an earlier attempt which one can view as a preliminary condensation of the latter. I think the explanations he offers are in general persuasive, with some deep reservations about his account of the facts of the Old and New Testament times.