The map most people carry is a century out of date. Christianity is still the largest religion in the world at roughly 2.3 billion, but it is no longer a European one in any meaningful demographic sense.
Its growth through the twentieth century was overwhelmingly in sub-Saharan Africa, where the Christian population rose from a few million to several hundred million, and in Latin America and East and Southeast Asia. Over the same period observance collapsed across most of Western Europe, which had been its heartland for a thousand years. More Christians now live in Africa than on any other continent.
That shift is not only geographic. The forms growing fastest, Pentecostal, charismatic, and the African-initiated churches that answer to no Western body at all, are less institutional and less textual than the traditions that dominated the religion when Europe defined it, and their theological centre of gravity is markedly different.
The consequence for anyone reading about Christianity is that most general accounts describe a European religion that, statistically, no longer exists. The doctrine in the creeds is shared; the practice, the music, the authority structures and the arguments are increasingly set elsewhere.