If you want a book about world history from about 0 to 1500, as processed through the lens of the spread of Christianity, you probably can't do better than this work. Moffett lays down the various theories about the earliest church, much of which we are essentially dependent on legends for. How much of any of these are true, we don't know. Then, he proceeds to explore the Nestorian and Jacobite faiths as they developed out of the more known portions of Christian history.
Christianity in the East at one time might well have had more adherents than in the West, but you wouldn't know it today. The history Moffett tells explains many of the reasons. Unlike in the West, Christianity in the East never had a major empire-wide political promoter--no Constantine--though there were occasional small kingdom adherents. As such, its popularity remained always at the mercy of whichever entity was in charge, as well as remaining often something seen as "foreign." Some kingdoms, such as that of the Mongols, were relatively open religiously, thus allowing Christianity to thrive as one religion among many, but others not so much. Thus, Christianity, rose and fell and rose and fell again throuough Asia, with the rise of various kingdoms and with the advent of Islam, whose influence grew stronger than in the West.
All that said, 1500 years across thousands of miles of land is a lot of material to cover. At some point, though accessibly written throughout, the work became for me more a set of names with a few highlights. I found it hard to keep track of all that was going on. Indeed, any history of nations outside the West, being less familiar culturally to me, tends to be difficult reading, a testimony to what we focus on in school. When more is unfamiliar, it's harder to find holds on which to ground one's self; were I to go read more and more histories of the East until events and people became easier to place, I'd probably find this book more accessible.