An incident from "Real Time with Bill Maher" from 2006 sticks out in my mind as an interesting case-in-point for one of the difficulties of discussing religion generally, but in particular in understanding the difference between creedal religions (like Christianity and Islam) and practical/experiential ones (like modern Paganism and polytheism). In that particular episode, Bill was joined by Sandy Rios, Reza Aslan, and Bradley Whitford, and they were discussing points related to the recent (at that time) documentary Jesus Camp and "religious fanaticism." You can see a clip of this section of the show here.
While there's a great deal that would be worth discussing (and correcting) in this particular segment, a comment by Bradley Whitford that starts at about 7:30 on this clip has stuck with me as a fundamental misunderstanding about religion. Bill Maher is arguing (just before this) that "people talk to God," but that there is no "God" to talk back to people, and so there's a gap which is then filled with human ideas, including ideas about killing people of other religions. Bradley Whitford (who identifies as a liberal Episcopalian) then said that "belief" bridges that gap, so that someone who says they "believe" that Jesus is the "Son of God" (like him!) is somehow reasonable (because, it is implied, of the uncertainty inherent in the notion of belief), whereas someone who says that they "know" Jesus is the Son of God is "schizophrenic." Thunderous applause and approval by Bill Maher follows this statement.
A great deal could be said at this stage about the atheist/agnostic context of these comments (as Maher is an atheist/agnostic), and a great deal more could be said about the differences between "orthodox" Christianity as it has come to be known over the last nearly-two-millennia and gnosis, as understood in other forms of early Christianity. But, gnosis and direct experience of the reality of different deities—or, to put it slightly differently and yet equivalently, of the reality of particular theologies—is at the very heart of polytheism, animism, and the religious sensibilities of many different traditions, including Hinduism, Shinto, all ancient forms of polytheism, and modern Paganism, amongst many others.
And yet, some of these very same types of debate are occurring right now in a variety of modern Pagan and polytheist groups. There is often an active disdain, perhaps masking a fear, of people who are mystics and who have direct experiences of various deities and other divine beings. It sometimes seems as though a sensibly ironic, distanced, philosophical, and overly-logical approach to religion is preferred in some of modern Paganism of various stripes, rather than the actual admission that yes, indeed, there are deities that exist and to which we are drawn.
I've also heard the statement from some that they are animists or polytheists, but they don't "worship" the gods or the spirits, as if worshipping or reverencing something greater than oneself is inherently wrong or demeaning. Again, I find this at great variance with many worldwide religious traditions that have no qualms about stating plainly and blatantly, with neither apology nor excuse, that divine entities deserve our worship. (The nuances and origins of the word "worship" would be instructive to investigate, but I will leave that aside for the present.)
There is, of course, a necessary difference between what can be understood as fairly "objective" knowledge, generally speaking, and the subjective nature of all religious knowledge. For example, we can state unequivocally that Christianity did not exist in the 7th century B.C.E. in Japan, and we can also state that there was no cultus of Disciplina in the Roman Empire until the reign of Hadrian in the early 2nd century C.E., because these are archaeological and historical facts. But, the beliefs (a term which I understand as "articulations of experiences of divine realities") that follow from these, and many other, religious facts are not necessarily the same for everyone. Not everyone's experience of Disciplina, or of Jesus, or of Odin, or of Lug, or of Hermes, or of Hathor, or of any other divine being is going to be quite the same.
This is where the rather useful concept of UPG comes into play in modern Paganism; and yet, simply dismissing someone else's legitimate and even transformative experiences as "mere UPG" is, in my view, not very useful nor very charitable toward our fellow humans. Someone else's UPG may not be binding for me, but it can certainly inform my own ideas, and even influence my own practices and experiences in the future. Indeed, everything written in any literate source or attested to in any archaeological object from the past is evidence of someone's UPG at some point, and the ways such "lore" has shaped and even built people's subsequent experience over the ensuing time periods cannot be discounted.