This is a great book with lots of evidence to back up the claims of Jenkin's thesis. The two biggest shifts that Jenkins talks about are the move of Christendom from North to South and liberal to conservative theology.
Jenkins argues that Christianity was never a Western and Northern religion. Before the fall of the Roman Empire, Christianity was in full swing. Moreover, Jenkins mentions that before 1200 A.D. more than half of all Christians lived in the Middle East and Asia. This alone is an amazing point. Based on this, he argues that this is what is happening now at a rapid pace. More and more Christians live in a non-Western world.
Second, Jenkins argues that with this "new face" of Christianity, these Christians are more traditional and conservative than the liberal western church. This, too, is an important point, because this conservative Christianity will probably run into conflict with Islam.
This is an important work of scholarship that shows that Christianity is changing.