It’s generally the more conservative, even fundamentalist, versions of religions that strenuously evangelize and seek converts.
So, other than bringing up children in the faith, how do more liberal religions reproduce themselves? I imagine there’s a good deal of stealing from more conservative movements. For example, a college student can come to think scriptural literalism is unworkable, and drift toward a less rigorous version of her childhood faith. There has to be some amount of conversions from nonbelieving circles as well; there has to be something to the stereotype of nonbelievers coming to look for some more spirituality in their lives.
Thing is, such speculation doesn’t take me beyond the image of liberal religion as a watered-down compromise between a full-throated faith and nonbelief. I suspect that isn’t good enough: liberal religion has its own integrity, its own attractions besides the image of moderation and truth being in the middle.
I should really talk to a sociologist of religion about this.