Afterword to Pilgrim's Regress

This seems to me, unfortunately, to fit into the category of Lewis' heavier writings that I cannot even follow. So many times he refers to British (I think) culture-specific things that I don't understand. Then he will just use some marvelous and smooth prose that I can't understand that either.

For example, he talks about how there are two extremes in so many areas. Then he consider theology, and how even that has two extremes. But when he describes them, he calls one extreme "driving out the bondmaid's son" and the other "quench not the smoking flax." Now what in the world do those two phrases mean? He does explain it more in the following sentences, but I can't understand them either. And this is just a sample. The whole essay is like this. I wish I could understand it.