I have a theory. It can only be that, as I haven’t seen any studies good enough to really assess things. But if we forget the fact that conservatism is so unexpectedly popular and look at Christianity as a whole, I think there are two separate questions:
* Why is Christianity becoming less popular?
* Why are people not going to Church as much?
These are separate questions, because during most of US history our population was almost all Christian, but participation in Church wasn’t as common as now.
There’s probably no easy answer to either. I would suggest that there are several considerations:
* There used to be no realistic alternative. Parents, teachers, politicians, writers, all were Christian. Except for a new free thinkers, Christianity was the only alternative they knew. That is certainly no longer the case.
What’s more, not all of the alternatives are atheist. There’s nothing evil about Harry Potter. But Harry Potter and a whole raft of other fiction provides us with things to think about, even ways of thinking about the human condition, that aren’t visibly Christian. It’s not like the days when people read the Bible or Pilgrim’s Progress because there the most interesting stories around, not because, like spinach, they are good for you.
* Church was the primary social outlet. That’s no longer the case. We now have all kinds of activities to be involved in. I work with youth. Even the ones who want to go to church find it hard to juggle their schedule. One kid who I really admire is into dance. His troop rehearsed Sunday morning. He went to Sunday School, left for practice, and came back when the service was ending for confirmation class. But that’s an unusual level of dedication. (He’s now an elder.)
* Hellfire doesn’t motivate people anymore. It seems clear from the little I know of the 16th Cent that hell was very real to people. The most popular books during that period were books on how to die well. People were willing to put up with a church that they knew was corrupt because it was the only source of grace to avoid going to hell. During the early American period, it seems like for many people Church was present primarily during revivals. But what did those revivals do? As far as I can tell, they reinforce the fear of hell.
It’s not that Christians today don’t believe in hell. All conservatives do, and even many liberals. But they don’t have the kind of fear of it that would cause them to do difficult or unpleasant things.
Of course there are also intellectual questions. Does science disprove Christianity? But those all have fairly easy answers. The problem isn’t that Christianity has been disproven but that people aren’t motivated to follow it. They have other places to go, other things to think about and talk about, and no motivation strong enough to overcome these things.
I think churches are finding it hard to find a way to motivate people to get up Sunday morning, and to develop hard virtues. This is not a liberal / conservative question. The same thing is happening to both of us.
So how do you motivate people? The situation was similar in Rome. That was also a culture with lots of alternatives, though perhaps the fear of hell was still real at least to the Christians. The only study I’ve seen on why Christianity spread (Rodney Stark) suggested that it was a visibly better way of life. Christians treated women and children better, they cared for the sick enough better to affect life spans. Friends recognized something they wanted to be part of.
I think we’re going to have to do something similar. Yes, we need a good intellectual foundation. We have to be able to justify what we teach and deal with doubts. But the Church has to provide a kind of community that people can’t get elsewhere.
That’s why you’re right that doctrinal harshness and moralism are going to be fatal. I follow the Christian Advice forum here. The number of accounts of churches that are just nasty places to be is shocking to me. A 16th Cent peasant might put up with that to avoid hell, but modern Christians aren’t going to. Even though logically they probably should. Is it unbiblical for people to judge us by the kind of community we are? I'm not so sure that it is.
To get back to the OP, I don't think mainline theology has much effect on all of this. It's probably the only thing that people with a certain kind of education will take seriously, but other than that, we have all the same issues. Being intellectually sound doesn't make one relevant, even if some people in the 1950's thought it was going to.