The problem is that there are independent justifications of liberal politics: modernity and perhaps Christianity. So how do we know what's doing the work in the liberal Western Christian's reasoning?

I'd have some specific questions for this Christian. First of all, to what extent does she adopt the historical-critical approach to reading the Bible? Does she think Jesus literally, physically rose from the dead? Is that belief based on faith or does she think there are good reasons for being a Christian? In short, how seriously does she take her religion? Is it mainly a social club? Is she just going through the motions because she's too modern and rational for her own good?

The most important difference between liberal and conservative Christians, I think, is epistemological. How naive or sophisticated and informed (modern) are they in reading the Bible? It's the sophisticated hermeneutics that lead to agnosticism or to atheism (i.e. to a very weakened form of Christian faith).