Reviewing (highly favorably) a book about a Jesuit missionary to Tibet, David Bentley Hart muses on the obstacles the Christian faith faced (and faces) in that country:

Cultural conversion is something that happens under only very particular circumstances. Christianity spread so relentlessly through the world of late antiquity in large part because it found itself in a hospitable climate of diffuse religious and philosophical longing, full of powerful and sophisticated forms of worship and schools of thought, but open on all sides to answers from other quarters. There was no single, coherent organizing principle at the social center of religious adherence, no embracing system of beliefs and values. And much the same seems to be the case today in those parts of the world where Christianity is making its most impressive inroads.
In Tibet, however, the situation was altogether different. There [the missionary father Ippolito] Desideri was attempting to enter into and transform a fully realized and complete culture, homogeneous in its spiritual and social constitution, and possessed of a refined and complex and utterly pervasive religious system, with its own metaphysical schools, mythologies, devotions, cults, and consolations. It was a wholly coherent culture, for all intents and purposes sufficient unto itself.
This is not to say, of course, that Tibetans are not prey to the same perennial questions that other peoples are; it is only to say that those questions are susceptible of a great diversity of answers, many of which may even be entirely false and yet still perfectly convincing and satisfying. And, whether based upon truths or falsehoods or a perplexing combination of both, Tibetan religious culture was, by comparison to many other religious cultures, a kind of closed totality, which no Jesuit (however idealistic) had much chance of breaking open.

It has long been observed that once a nation goes Islamic, it doesn’t turn back. I think this fact must have something to do with the dynamic that Hart observes at work here — the totality of the religious system rendered Muslim populations impervious to an appeal from an alien religion. I’ve highlighted before ex-missionary Daniel Everett’s fascinating experience with an Amazon Indian culture, in which he came to understand that they were psychologically incapable of taking the Gospel seriously, because of their eccentric epistemology. I think as well that we in the West are, on the whole, culturally unsuited for Islamic conversion. But nothing is forever. It took hundreds of years, and lots of violence, to de-Christianize Europe, but here we are. Humans are naturally religious, and Europe will not remain a secular island in a religious world for long (demography will have its role in that). The question is not whether Europe of the future will be religious, but what sort of religion will it accept? A reborn Christianity? Islam? Or an ersatz secular religion, a la Nazism or Communism? For the answer, one should probably start looking at the cultural dynamics D.B. Hart identifies in his essay on why Tibet is not Christian.