I just finished this book by Paul Johnson, and I do recommend it. I have read quite a few books on the same subject, but this one has a rather different perspective from the others.
Following the book's progress, Christianity's history is a series of great strides, followed by "bozos" (my term) coming along to mess things up. Read it and see if you don't agree.
For Unitarians, there is one interesting conclusion that Johnson makes that I have never heard before this. He believes that Islam made easy headway into North Africa, Syria and nearby areas, because many people there were never comfortable with the Trinity doctrines and were much happier with a more truly monotheistic religion. Food for thought.
I also found his take on Christian missionay work in Asia, especially Japan, and what might have been, really interesting. Johnson is British and his take on American Christianity is a new one on me; I think I agree with him. I don't want to spoil it with an inadequate summary, so I won't say any more here.
The book seemed to start off on a positive note, but got progressively more negative in tone. The Epilogue did go to a positive conclusion and was hopeful for the future. Johnson has a lot of good to say about Christianity here, and the book should be viewed in this context I think.
One drawback of the book is that there is a lot to cover in 500 or so pages, and the books seems to gloss over certain events. There seems to be more detail on the early years.
The book was written in the 1970's, but a new edition was brought out recently. Give it a try.