Benjamin, thanks for the thought provoking article. I love reading articles that make me think. Considering that I'm a Lutheran pastor who loves the music of Nirvana, you had me hooked right away. I highlighted your statement "No mainstream culture could ever be conceivably Christian." That's an understatement. I'm reminded of Jesus' words recorded in his discussion with Pilate - my kingdom is not of this world. Any time we Christians think we can try to make a culture Christian, we're missing the point of what Christianity is, and what it means to follow Jesus and what he is about. But I think that's because for many Christians, their faith is rooted in a fourth century plus version of Christianity that is married to empire - Christendom.
As far as I'm concerned, that's a different religion all together.
At any rate, you pose an interesting question at the end - what is the meaning of Christianity in the 21st century? I ascribe to the idea that every 500 or so years there is a major "reformation" or reordering that happens within Christianity and that it is occurring now. Which means that it's too early to fully answer that question. Or maybe the answer to the question is this - the meaning of Christianity in the 21st century is a cleansing of itself, a sort of "repentance" of itself. Repent means change at its core. A turning. Christianity needs to do a hard look at itself and decide what it is and what it isn't. Maybe another major split is coming? Maybe some sort of dying off of denominations? Maybe a coming together of denominations that can finally play together in the sandbox of social justice? Maybe something else? I don't know. Usually these things time decades to sort out. The last reformation in the 1500's took most of the century, were messy and destructive. I just hope this one isn't as destructive. Messy I can live with.