My experience in life with self-identified Christians has largely been in the context of those people disagreeing, on moral grounds, with actions I take or people I support--almost always citing religion in their reasoning. It's possible the people I've dealt with just aren't components of this "mainline christianity" you're familiar with, but they use the same tools to believe these things. To me, those tools are egregiously flawed, and I have a vested interest in making sure those tools don't get used to believe false things that bring harm to myself or my neighbors.

> Because on the main point, they're all pretty much aligned...

Historically, wars have been fought over these disagreements--both within only Christianity, and in the wider religious space.

I think it's worth considering other religions personally because it's what led to my de-conversion: I couldn't answer the question of why, other than being raised in it, I should believe Christianity over, say, Buddhism or Islam. As I regarded other religions with skepticism, when I was a Christian, I should also regard Christianity.

While the specific point I've made previously deals with selecting a denomination and a church within that denomination it's also true that people choose religions for similar (flawed?) reasoning.

Further:

> What you see as fundamental differences are really more social than theological...

I'm not sure I'm convinced on this. Take gay marriage, for example: I vividly recall being 12 in our church, sitting in on a conversation between my (single) mother and our pastor, on how to deal with people who chose to sin in our lives, specifically referring to my father who was openly gay at the time. The church we went to was firmly against homosexuality, but was of the love-the-sinner-hate-the-sin cloth. On the upshot, they were relatively kind to those of the LGBT community, but they did still make it clear they did not support their "choices" and largely ostracized them--with reasoning that, in their view, was ensconced in theology.

While LGBT rights have certainly been a social issue throughout the world, I think dismissing this "difference" between my church and the one on my college campus who made a point of welcoming LGBT members is to minimize these actual theological differences. There's part of me that wonders if this is a bad-faith maneuvering (not on your part, but organized religion as a whole) to downplay socially repulsive beliefs without having to sacrifice their supposed moral authority.

> I think most mainline Christians would reject the notion that the Old testament is a factual historical record.

This certainly hasn't been true across history, and even now I harbor doubt. Perhaps I've only dealt with more fundamentalist types than you, but the opposite has been true in my experience, and is definitely not true of the more loudmouthed Creationist/Ken Ham style evangelicals. While they may not be representative of the majority, *they are affecting policy* in many regions of the country. My mother, an elementary teacher, frequently voices her frustrations that she's not allowed to pose creationism as an "alternative" to evolution in her classes science units--something that is allowed in several other states[1, though from 2014].

You see similar flaws in other arenas, too: my grandparents view climate change as an issue outside of human concern, squarely in God's hands, in part because they believe in life-after-death and the eventual rapture, so while they should do reasonably well to steward the planet, they don't think we're going to be here forever so it doesn't matter if Earth becomes an unlivable rock; while some may suffer the effects of an adverse climate, it won't matter when everyone's in heaven.

I also wonder about what motivates these changes in how doctrine is viewed. Supposing your right, what drove the digression that the Old Testament is not factual? I doubt it was the Church deciding on its own, outside of societal pressure. I'm sure it's because of pressure from those who found fault in the Old Testament teachings--those who were condemned by it, or ostracized by the churches of their time, and the Churches granted this concession without wholly usurping their power. But what about the next issue? Maybe folks are believing less in Noah's Ark, but how will they contend with folks who're trans, or polyamourus, or take issue with abstract (i.e., not historical) teachings of the bible like not rebelling against kings, for they have been ordained by God [2]?

[1]: http://www.slate.com/articles/health_and_science/science/201...
[2]: https://www.biblegateway.com/passage/?search=Romans%2013&ver...