A comprehensive demographic study of more than 200 countries finds that there are 2.18 billion Christians of all ages around the world, representing nearly a third of the estimated 2010 global population of 6.9 billion. Christians are also geographically widespread – so far-flung, in fact, that no single continent or region can indisputably claim to be the center of global Christianity. A century ago, this was not the case. In 1910, about two-thirds of the world’s Christians lived in Europe, where the bulk of Christians had been for a millennium, according to historical estimates by the Center for the Study...