Although not strictly useful for apologetics, The Next Chritsendom is, like Jenkins' past work I read (Hidden Gospels), a delightful read, especially if you have seen (as I have) how Western Christianity has lost anchorage in a contextualized meaning of the Scriptures. It is also useful the next time a critic boasts how so many people in the West are abandoning Christianity. The bad news for such is that their narrow sense of the world as ending in the West will come back to bite them: Christianity, Jenkins shows, is growing (and so is Islam, but not as much) at a furious pace in the global South (Africa and Latin America in particular, but also elsewhere), such that we will see a "worldwide boom"  of Christian growth in the 21st century. One can just hear Robert Ingersoll turning over in his grave -- and being turned like a roast in that grave by those "ignorant inhabitants of Central Africa" as he called them, who are one of the groups who will be providing the basis for that boom.
Even worse news for the critics: This new global Christendom is decidedly on the conservative side. It stands for traditional values, has no liking for separation of church and state, and also tends to be on the charismatic side. The prospect of billions of new Pat Robertsons ought to have a lot of Skeptics shaking in their boots; they have proudly claimed the West as an ideological triumph, even if too early, but as they have done so, the South was sneaking up on them and may just pull the rug out from under them, for as Jenkins notes, the new Christians have seen how we have lost our faith, and are now sending out missionaries, ironically, to set us straight.
Jenkins' work is also fascinating for its accounts both of early Christian missionary work in the global South (sadly, often a wasted opportunity after initial successes were not properly followed up on) and of modern martyrs for the faith that for some reason don't make the nightly news the same way that single deaths of celebrities do. As in Hidden Gospels, Jenkins has a few words for the media about their disinterest in such things. At the same time, Jenkins wisely advises us that with this booming Christendom will come certain political and theological challenges. Christian states will find themselves at odds with Muslim states, potentially producing more wars. And yes, some portion of the New Christendom will fall into doctrinal peculiarities; with the core will come also the cults.
Thus we can view the news of The Next Christendom with a joyous caution. We may rejoice that atheism will not be what it is cracked up to be; but we also need to see this boom as an opportunity for the Western church that remains -- with its two remaining advantages of wealth and access to knowledge -- to join the wider Christian community in a cooperative fashion, neither lording over it in colonial fashion, nor sitting passively by on that score while getting all in fumes over the Skeptics taking away our Christmas displays. Not that the latter ought to be neglected; but not at the expense of the former. To fulfill the Great Commission requires the Body of Christ to function truly as a body of one, and the coming of global Christianity represents a unique opportunity to reclaim the Kingdom's ground -- in the West as well as abroad.