The thesis, or main theme, of The Next Christendom by Philip Jenkins is the projection of Christianity's expansion in the 21st century. The author's theories are based on research he conducted using historical data, specifically conversion and expansion trends in Christianity. The author posits that the Southern hemisphere—particularly African and Latin America—will make up half of the Christian population in the next few decades.

The other theme of the book is Christianity's tense relations with Islam, which to this day is still has the fastest-growing membership of all religions.

Jenkins also predicts that various Christian sects will become more competitive with each other. This is reminiscent of the past figurative and literal wars between Protestants and Catholics, as well as the creation of splinter Christian sects such as Lutherans and Seven-Day Adventists.