Varieties of being Christian, or is there really a universal religion?

Probably because this is the Christmas season, I have decided to write about something that's been on my mind for quite a while. Many people who know me know that I enjoy reading the works of C. S. Lewis, and I think I have read most of his works that don't have to do with literary criticism: in other words, I've read most of his fiction and his pop-theology (he himself denied that he was properly a "theologian").

One of Lewis's most famous pop-theology works is Mere Christianity. In this book, he sets out to explain what he believes is the essence of Christianity, hence the word "mere": Christianity stripped to its bare essentials. (He also sets out to make a not very convincing proof of the truth of Christianity, but that's the topic for another post.) If I read and remember correctly, the main, Christianity requires the recognition that

1. Humans are depraved and subject to self-worship (Pride). (Lewis does not believe that humans' depravity is total or else, as he says, they would not know it.)
   
2. Humans' subjection to pride is inherent in their nature and leads them ultimately to spritual death.
3. Only Christ--through the free gift of salvation (grace)--can remedy this condition.
   

Now, two further points that Lewis makesL:

First, he does not insist that one must profess Christianity to attain salvation. Even though he believes that Christ is the only way, he is open to the possibility that others might be saved by Christ without knowing him.

Second, he claims that other world religions actually are approximations to Christianity. He claims that other religions shed their peculiar characteristics and become more "Christian" as they progress. In another work, Abolition of Man, Lewis appends a collection of moral sayings that he claims demonstrates that all humanity follows more or less the same ethical rules.

It's this second claim I'd like to interrogate further. But first, I'd like to discuss what I consider the "varieties" of being Christian so as to avoid mixing terms. Here they are, in rough order from less to more "spiritual":

1. Growing up in a Christian society (which for the sake of convenience, I'll define as a society in which an overwhelming majority of people self-identify as "Christian") makes one "Christian" in one sense, even if one does not profess Christianity. They are compelled to observe, or endure, Christian holidays and are compelled to undergo the casual references to Christianity-as-the-true-religion: the "in God we trust" logo on our currency; politicians' frequent invocation of a God that is almost always assumed to be Christian--even though they give a shout-out to Judaism and Islam; the sometimes transparent/sometimes not quite so transparent Christian messages.
2. Being raised in a Christian tradition also makes one "Christian," even if that "one" later rejects Christianity, i.e., converts to another religion or moves into atheism or agnosticism.
3. Professing "Christianity" makes one Christian in another sense. (I'll not go into what "truly" counts as Christian; it's too complicated and I'm too ignorant to do it well.)
4. Adhering to a belief system that somehow incorporates "essential" elements of Christianity. In this sense, to use Lewis's thesis that non-Christian religions approximate Christianity, people who don't profess Christianity approximate it in their beliefs. (Also, arguably, people who profess or self-identify as Christians but whose beliefs and actions do not approximate whatever is the essence of Christianity are not "Christian." Still, I'm not going to go into which denominations are and are not accurately called Christian.)

The last point is bothersome. Why say that other religions "approximate" Christianity? Why not say that Christianity, for example, "approximates" Hinduism and that as it sheds its conception of God and salvation it becomes more like the "true" way of Hinduism? Or any other religion. Heck, maybe even some sort of humanitarian atheism is the ultimate way and Christianity "improves" as it approximates that atheism.

Or maybe there is one universal "right" that all sincere religions "approximate." Of course, once I use the word "sincere," I get into trouble. Who is to judge sincerity? What standard is one to use in so judging? Still, I'll not go into that, at least not here. (I'm rejecting the possibility that there is more than one "right way." I do so partly for the sake of argument, but also because it seems to me that the notion of a singular "right" inheres even in the pluralistic notion of "all religions are equal.")

Since the notion of a universal "right" (nb: I'm not using "right" in the sense of, say, the "right to freedom of speech," more in the sense of "truth"; in fact, I should have used "truth," but I don't want to rewrite everything :)) is the last possibility I mentioned, it's obviously--following traditional expository style--the one I want to endorse, at least tentatively.

I hope to write later on this topic. But for now, I'll leave myself--and anyone who happens to be reading this blog--with some problems and challenges that this notion brings up:

1. It does not suffice to say, for example, that Christianity can be interpreted to sound like, say, Buddhism. It's probably possible to interpret any belief system and find elements common to other belief systems.
2. If I claim to identify certain "truths" that appear to be common in all major religions (the word "major," like the word "sincere," gets me in trouble here, too), maybe all the religions are wrong and there is another truth that they're missing or that they pervert irreparably.
3. What counts as a religion? Is Spinoza-like pantheism really a religion in the sense that, say, traditional Christianity is? Is the Daoism (Taoism) of Laozi really on the same level as, say, Islam, or vice versa? Is it even on the same level of the "popular" Daoism that appears to have emerged in China during the first millenium A.D.? Is atheism or agnosticism a religion? Is there such a thing as one atheism or one agnosticism, or are there variants?