In very broad terms, the subject of The Next Christendom by Philip Jenkins is Christianity. In more specific terms, however, the subject can be seen as the future development of Christianity, what it will look like in the decades and centuries to come.
Combining historical data with contemporary demographic information, Jenkins puts forward the thesis that the phenomenal growth of Christianity in the Southern Hemisphere over the past century or so hints at its future status as a genuinely global faith.
At various points in the book, Jenkins is at considerable pains to point out that, contrary to what many of us in the West may think, Christianity is not a Western creed. On the contrary, it is a universal faith that has taken root in a bewildering array of cultures across the globe.
It is this universal nature, the fundamental essence of Christianity, that Jenkins argues has been revived by the spread of the faith throughout the Southern Hemisphere. Jenkins does not doubt that, in due course, this process will only continue at an ever more accelerated pace, leading to the manifestation of what he calls “The Next Christendom,” a Christian world which is bigger, more diverse, and, to some extent, more conservative in both doctrine and practice.