This post from Faith and Theology makes some very important points about historical theology. Essentially, our task in reading theology from the past is not simply a matter of sifting out what is trans-historical and what is culturally bound. When we do that we assume that we can tell what is core and what is not. Instead, we must acknowledge the integrity of the text, and then realise that it was possible to write this theology (whatever it was) and be Christian. This way the writings of the past can challenge us (for instance, those silly bits that we sift out might be quite challenging when worked theologically), and we can no longer assume that we possess the only way of looking at things.
The author critiised in the post responded later to the accusations. He agrees with the points made by Ben Myers, but just doesn't think he made the errors as accused.