Before I begin, I'd like to note that there's a significant difference between evangelical Christianity andfundamentalist Christianity. There are evangelical fundamentalist Christians, but the two are far from synonymous. Fundamentalism holds to Biblical inerrancy, and as I'll describe, that's hard. Evangelicalism is based in personal conversion, sharing the gospel, and Biblical authority. It does not require, as fundamentalism does, literal interpretations of scripture.
So, evangelical and liberal Christianity are too easy to persist. Religious belief, if it is to be transmitted consistently, relies on several factors. The one that I'm focusing on here is difficulty. The religions that have persisted for long periods of time are very hard to follow. On one hand, they require conspicuous and, critically, impractical behaviors as professions of faith. For example, the following from a Jewish rabbi:
That most of the Kashrut laws are divine ordinances without reason given is 100 percent the point. It is very easy not to murder people. Very easy. It is a little bit harder not to steal because one is tempted occasionally. So that is no great proof that I believe in God or am fulfilling His will. But, if He tells me not to have a cup of coffee with milk in it with my mincemeat and peas at lunchtime, that is a test. The only reason I am doing that is because I have been told to so do. It is something difficult.
Similarly, Islam requires that its followers stop what they are doing five times a day to pray, never mind how inconvenient or poorly timed or indeed dangerous doing so might be. Catholic ritual also fits the bill; it is very elaborate, very strict in form and slow to change over the years, and very definitely required. In Cardinal Ratzinger's Declaration "Dominus Iesus: On the Unicity and Salvific Universality of Jesus Christ and the Church", it is stated over and over the things that Catholics must "firmly believe" (and those italics are in the original), but it also switches several times to what "the Catholic faithful are required to profess". And these are merely examples we can find within the Abrahamic religions.
These actions are important precisely because they are hard to do. They allow the believer to feel that they areproving their faith, simply because they are choosing to follow commands that they, frankly, don't understand. There's no reason for it, there's no practical purpose, it is done because it is commanded, and thus demonstrates one's loyalty to the religion from which the commands issue.
On the other hand, they require people to believe in things that are incomprehensible. Again, this is hard to do, because it is a natural human tendency to try to figure things out. Dan Dennett calls this "credal athleticism", the boast that my faith is so strong that I can embrace a bigger paradox than you can. This serves much the same purpose as the conspicuous actions noted above; believing something that is patently ridiculous is hard to do, and getting to the point that you can do it feels like you're accomplishing something difficult in the service of your god. Indeed, it may even become one of those conspicuous actions when those beliefs are professed. When a fundamentalist Christian loudly and unwaveringly proclaims that no matter how much evidence you show him, he will continue to believe that the Bible is literally true, you may throw up your hands in exasperation, but he feels that has accomplished a very difficult and conspicuous task, that of blatantly professing paradoxical ideas, and that's a good feeling for him.
Incomprehensible doctrines have another benefit, as well; they must be passed down verbatim. Paraphrasing won't work, because to paraphrase you have to understand what is being said. For the incomprehensible, the only thing to do is directly quote the doctrine. This expresses itself in devotion to hearing, reading, and memorizing scripture, and in exposing ones self to theological tracts and doctrinal statements that are quite simply opaque and impenetrable ("I don't understand what the Pope meant, but here's what he said"). It even plays into the idea, common in many religions, that god is an ineffable, unknowable, profoundly apophatic entity. God is "the greatest conceivable being", "god is love", god is "the ground of all being", god is "ultimate reality"; trying to dig into what these statements actually mean is often a pointless exercise, and so they are just said because there's no deeper understanding of them to be had.
But liberal and evangelical Christianity is different. It places a focus on what the words of the Bible mean to you. There's an emphasis on personal revelation, personal conversion, personal experience of god. Services include PowerPoint presentations, contemporary music, a dress code of "whatever", and no mandatory attendance. Some churches refuse the word "sanctuary" because it's too religious. Churches spend time not on issuing doctrines, but providing daycare services for their members. We're all familiar with the signs: definitions of "god" that depart radically from classical theism, the "it's not a religion" language, consistently relating stories about personal experience of god, novel interpretations of scripture, cherry-picking, etc. The only thing that is demanded of such believers is that they spread the gospel, but they do so in whatever manner they see fit, often without quoting a single verse of scripture.
This is way too easy. Consistent transmission of religious beliefs depends very strongly on the formulas being passed down intact, or at least almost exactly so, and the limitations on mutation of ideas imposed by strict rules of behavior and incomprehensible doctrines are almost if not entirely lost in contemporary evangelical and liberal Christianity. I've argued before that in some cases, people who call themselves Christians, but who are part of these groups, are no longer following anything that can be reasonably defined as a religion. It has changed so quickly that only a few vestiges of the original forms are left, and the actual beliefs no longer resemble the model on which they were based. Thus, I think that such forms of Christianity cannot persist in the long term. Their only consistent basis is a reliance on the Bible as a source of wisdom, and for many it is rapidly becoming secondary to one's own personal experience of "what god wants". While I, as an atheist, most definitely disagree with the more rigidly structured religions in terms of whether or not they are true, I cannot deny that they have traits which give me more confidence in their "staying power".