I don't much enjoy discussing religious matters with adherents of doctrinaire faiths such as the majority of Christians and Muslims (there are exceptions in both cases, of course), except when the mischievous imp in my personality takes over and prompts me to yank their chains. We come at spirituality, the doctrinaire and I, from completely different epistemological assumptions. It is therefor extremely difficult to find common ground or any basis for discussion. But examining these two diametrically opposed ways of approaching the truth can perhaps be useful and instructive.
My own approach is one of discovery. I hold the sacred reality which is variously (and always metaphorically) termed God, the Gods, or other words for principles underlying the reality that we sense and in which we live, to be something that, while never possible for the human mind to encompass in its totality, we can discover. It lies at the bottom of our own being and at the core of the world's essence. The process of discovering it is also a process both of self-discovery and of self-transformation, in which the mind grows wider, deeper, more perceptive, and better able to embrace and understand. The discovery of the sacred is a stretching of the mind. It's the only thing that allows sacred reality to be comprehended at all, in my view.
Since sacred reality is there to be discovered, everyone who discovers it finds the same reality, even though the process of discovery varies. As an old Japanese proverb has it, there are many paths up the mountain, but the view of the moon from the top is the same. This is why mystics of all sorts, no matter what their starting-point religion, always end up saying very similar things and recognizing the oneness of all faiths.
I view spirituality as like a wheel, in which sacred reality is at the hub, while normal consciousness resides out on the rim. Religions are like the spokes of the wheel, bridging the rim and the hub. Each spoke is most distinct from the other spokes at the rim, at the shallow end of perception, and approaches common ground the closer it approaches the hub of the wheel, where God is.
All of this makes perfect sense to me, but it rests on the epistemological assumption that discovery is the way to know sacred reality. The doctrinaire make a different assumption: that sacred reality cannot be discovered, but can be known only by revelation.
Revelation is not a human act but a divine one. It involves sacred reality revealing itself to a prophet such as Moses, Jesus, or Mohammed, who communicates this reality to others in words and symbols. The more thoughtful among the doctrinaire recognize, as I do, that ultimately the sacred reality is unknowable, but they disagree with me that it can even be approached through human effort. The inadequacy of revelation for complete knowledge is acknowledged, but of course the same is true for discovery; when it comes to sacred reality there is no such thing as "complete knowledge."
An epistemological divide is essentially unbridgeable, because neither side acknowledges the proofs of the other as valid. Each side has its own way of knowing, and hence its own way of proving. There are perhaps some observations I can make about problems with the revelation mode of knowing, but when people begin with the assumption that a certain body of written word is revealed truth, there is not much more to be said, is there? Still, the effort should be made, if nothing else to clarify my own thoughts.
One problem with revelation is correctly identifying genuine revelation and distinguishing it from pretenders. Consider the Bible, for example. This is a collection of short books in three different languages (Hebrew, Biblical Aramaic, and ancient Greek) by many different authors. At some time in the dim past, Jewish authorities have identified the five books of the Torah (Genesis, Exodus, Leviticus, Numbers, and Deuteronomy) as sacred, with lesser degrees of sacredness applied to the Biblical historical accounts, the five books of poetry and philosophy (Job, Psalms, Proverbs, Ecclesiastes, and the Song of Songs), and the books of the major and minor Prophets. The Imperial Church at its founding in 325 or shortly thereafter adopted all of these books from the Jews without distinguishing among them in Jewish fashion, simply saying that they were all equally sacred, and added a selection of Christian writings chosen from among the many in circulation at the time: four Gospels, one history of the very early Church, a number of letters of instruction from Paul of Tarsus and several of the Apostles to various Christians, and one (very weird) book of prophecy. In each case, we have the testimony of various authority figures, backed by the government either of the Kings of Judea or of the Roman Empire, that these writings are to be taken as sacred.
So the first question is simply this: Why should we believe them? On what basis should we conclude that politicians (as we must regard these men) are proper judges of spiritual validity? Today's politicians certainly don't inspire a lot of confidence along those lines. I mean, would you accept the word of President Obama that a particular text revealed the truth of the Gods? Not I, and I say that as someone who voted for him. I voted for him for President of the United States, not for Supreme Spiritual Leader and Prophet. Nor would I consider him an appropriate choice for the latter office.
If we cannot trust the enlightenment of those who have chosen the sacred books, how can we take their word for it that the books are sacred? How can we be sure that they chose correctly among the various possibilities?
Another problem arises when we examine the texts of alleged sacred books themselves, and I am not referring here to contradictions or clearly non-factual statements (those are open to interpretation or to the recognition that sacred writing is neither science nor history). I am referring to cases in which the alleged revelatory text seems to itself endorse a discovery approach. Many of the parables of Jesus seem to point that direction, as does his encouragement to the Apostles to develop their own relationship with God, to recognize the presence of God within them, and to draw upon this presence through faith to accomplish miracles similar to (and even greater than) what Jesus himself is said to have done. Jesus himself seems to have endorsed a discovery method for identifying sacred truth, so isn't the allegedly sacred text itself contradicting, here and in other places, the claims of those who swear by it?
Finally, let's recognize that all sacred writings had human authors. Each human author, if the claims of the revelationists are correct, had a direct relationship with and/or experience of sacred reality, from which he derived the written word. As this sort of relationship and experience is what discovery advocates like myself are talking about, revelation draws upon the same source -- but at second hand rather than directly. If discovery cannot suffice to gain the truth, then neither can revelation, because revelation depends on discovery ultimately.
All of these arguments seem good to me, but I know very well that they will not persuade the doctrinaire, because the application of logic and evidence are themselves inappropriate to a revelation-based epistemic model, except insofar as logic reasons from the premise that the text is sacred, and evidence draws upon the words of the sacred writings themselves, without questioning their validity (only their interpretation). It is in the end a pointless exercise, except insofar as it helps our own thoughts to be clear.