In the past, I've commented on France's ban of the niqab, noting in passing that the distinction some people make between religion and culture (emphasizing, for example, that the niqab is a Middle Eastern cultural tradition rather than a specific Islamic law), is irrelevant to question of whether a woman has the right to wear whatever the hell she wants in public - including, of course, a niqab if that's what floats her boat.
At the time, I more or less glossed over what I thought the actual distinction was between culture and religion, so with that in mind I'd like to expand a little on these thoughts and make the further claim (not so very controversial, I think) that the line between religion and culture tends to be blurry at best and, at worst, damn near non-existent.
Take Christmas, for example. Christmas, in most of the western world, is celebrated on December 25th. Is that a religious or cultural fact? Note that, hard as you might try, you will not find a date for Christmas in the Bible.
How about the Catholic admonition concerning the consumption of meat on Fridays? Again, this prohibition doesn't exist in the Bible, and it's not even followed in every variant of Christianity. So does that mean it's a cultural tradition as opposed to a religious one?
How about a nun's outfit? You know, the penguin suit? Culture or religion?
Adam and Eve ate an apple from the Tree of Knowledge and were consequently banished from the Garden of Eden, right? Wrong, as it turns out - nowhere in the Bible does it say they ate an apple. So, this business of the apple - culture or religion? Maybe tradition? Is there a difference?
Would you classify Paradise Lost as religious literature? That story, about the fall of Lucifer and his subsequent transformation into Satan, is not told anywhere in the Bible. There is certainly a well-established Christian tradition surrounding the fall of Lucifer, but it's not what you'd call dogma.
Are you detecting a theme? If you're going to argue that a particular practice or tradition only counts as "religious" if it's explicitly outlined in the holy book of said religion, you're going find that the vast majority of practices or traditions that one would normally and rightfully think of as religious are, in fact, anything but. The lesson one should take from that observation, in my opinion, is that it's ridiculous, in many cases, to draw significant distinctions between religion, culture and tradition.
A practice is religious if it forms an essential part of your religious life. If the practice or tradition is encouraged by a member of your religious leadership, if the norms of this practice or tradition are maintained by said religious leadership, if you are encouraged or forced to participate in this practice or tradition on pain of religious or social persecution (one and the same, in many cases), then said practice counts as "religious", whether or not it's explicitly laid out in your particular holy book.
So, avoiding meat on Fridays? It's a religious practice because it is encouraged and prescribed by Catholic authorities. Covering your face in public? Also a religious practice, even if the Koran is notably silent on the subject.
Now, I do understand that making a distinction between Middle Eastern culture and Islam is often simply an attempt to combat Islamophobia, and I do sympathize in spirit, but I think there are better ways to do that. One first step would be to simply acknowledge that there are, in fact, variants of Islam which advocate forcing a woman to cover her face in public, and to be clear that one disapproves of such variants. You can, of course, be crystal clear that not all variants of Islam advocate such practices, but trying to claim that they aren't part of at least some of them is analogous to saying that the tradition of avoiding meat on Fridays is not part of Catholicism. It's a hair splitting exercise that serves no one.