The Problem of "Public Revelation"
"It is a contradiction in terms and ideas, to call anything a revelation that comes to us at second-hand, either verbally or in writing. Revelation is necessarily limited to the first communication — after this, it is only an account of something which that person says was a revelation made to him; and though he may find himself obliged to believe it, it cannot be incumbent on me to believe it in the same manner; for it was not a revelation made to me, and I have only his word for it that it was made to him." -- Thomas Paine, The Age of Reason (source)
In the Catholic tradition a distinction is made between "public" and "private" revelation. "Private" revelation is easy enough to understand, but the "public" variety strikes me as somewhat problematic.
These terms are not meant to indicate to whom the revelation was made, but rather for whom the revelation was intended. In the case of "public" revelation, the answer is, apparently, "everybody."
This revelation intended for everybody is said to be contained in the deposit of faith. This is a series of supposed facts to which people are required to give their assent -- according to official church teaching, that is.
According to the Catechism, "we believe [revealed truths] 'because of the authority of God himself who reveals them, who can neither deceive nor be deceived'" (§156). Similarly, the following paragraph asserts that faith (meaning assent to revealed truth -- cf. §150) "is more certain than all human knowledge because it is founded on the very word of God who cannot lie" (§157).
These are curious statements. Personally, I've never heard of anyone rejecting a supposed "revealed truth" because they thought God was being deceptive -- if they reject it, it is because they don't believe God revealed it. The honesty of God isn't really the issue.
The issue is, which truth claims, if any, are revealed by God? Between competing truth claims, how is one to decide which ones to believe and which ones to reject?
The Catechism offers some suggestions: "the miracles of Christ and the saints, prophecies, the Church's growth and holiness, and her fruitfulness and stability 'are the most certain signs of divine revelation, adapted to the intelligence of all'" (§156).
So, supposedly these are criteria by which the truth claims of a religion can be measured: a) miracles performed by the putative founder and some subsequent adherents; b) the growth, fruitfulness and stability of the religion; c) the holiness of its adherents.
Let's look at these individually.
A. Miracles Performed By the Putative Founder and Some Subsequent Adherents
This is a problematic criterion, for several reasons. First of all, as anyone who has studied world religions with any depth can attest, Christianity is not unique in this regard. Miracle stories in non-Christian religious traditions are a dime a dozen. Secondly, there is the fact that these are only stories. They can hardly serve as an argument for believing religious truth claims, because they are themselves religious truth claims.
B. The Growth, Fruitfulness, and Stability of the Religion
As far as growth is concerned, there is no question that the story of Christianity is an impressive one. It certainly overtook Judaism in a hurry -- of course, the "no circumcision" thing gave it a pretty serious edge when it came to making converts. There is the problem of Islam, however: it's nearly six hundred years younger than Christianity, and yet it is expected to overtake Christianity in terms of numbers this century. So I don't know if Christians really want to play the "growth card" -- it's an argument with a fast-approaching expiry date.
Fruitfulness? I don't know what is meant by that.
Stability? I've studied church history. I don't know if "stability" is a word I would use to describe it. At any rate, I don't see how any of these can be considered "certain signs of divine revelation."
C. The Holiness of its Adherents
Have Christians, in general, demonstrated greater holiness than Buddhists, in general? Or Hindus? Or the adherents of any other religion?
John Hick (a theologian that I admire, even though I disagree with him a lot), suggests that this is often assumed. "But," he says, "as a factual claim this is extremely dubious. In fact, I would say that it is manifestly false. It would certainly be most unwise to let the Church's claim to unique centrality stand or fall by it" (source).
It's a curious argument, in any case -- especially considering the fact that when the sins of the church (the centuries of persecuting non-believers and heretics, the Crusades, etc.) are used as an argument against the truth of Christian beliefs, conservatives immediately insist that the behaviour of past Christians has nothing to do with the validity of their beliefs. In other words, holiness among believers is an indication that their belief system is true, but sinfulness among believers is not an indication that their belief system is false. There's something fishy going on with that.
The Limits of Reason
I don't think "reason" can demonstrate, or even suggest, that traditional Christian truth claims are clearly true in a way non-Christian religious truth claims are not. Which raises the question: if one cannot decide on the basis of reason, is one's decision to believe them not arbitrary?
One of the leading philosophical defenders of Christian exclusivism is Alvin Plantinga, a Calvinist who teaches at Notre Dame. He insists that it is not arbitrary because the believer does not see his or her beliefs on an "epistemic par" with the beliefs of other religions. Which is a classic demonstration of "how to respond to a question without answering it." We only need to rephrase the question: if it cannot be determined on the basis of reason, how does one determine that one's beliefs are not on an epistemic par with the beliefs of other religions? 
The Irrelevance of Dogma
In a previous post, I said that progressive Christianity, as I conceive it, is primarily interested in undoing the "inauthenticity" of the tradition, returning to the ideals found in the teachings of Jesus, and disposing of anything that obscures those ideals. The emphasis on doctrinal orthodoxy -- that is, in "believing revealed truths" -- falls in the latter category.
I am not suggesting that beliefs are not important. The belief that all humans are created in the image and likeness of God is, I would suggest, an important belief. It can actually influence the way you live your life, if you strive to act in accordance with it. The belief that justification is by grace would mitigate against works-righteousness, if it was understood and actually adhered to. But that's a subject for another post.
Dogmas are irrelevant. I often find people arguing over whether or not Jesus was conceived virginally, but I don't think this is really the issue. The bigger issue is, why does it matter? How could it possibly be important that people believe it?
Instead of arguing with conservatives over whether or not a particular dogma is "true," wouldn't it be more fruitful to ask them why they think it even makes a difference? My guess is, they won't have a coherent answer to that. At the very least, they won't be able to answer that without identifying church authority with divine authority, which is an inevitably circular (and therefore fallacious) argument.
 The quotation from Plantinga is from note I made in my journal a few years ago. I'll try to find the exact reference later.