A reader writes:
I agree with your reader that there is no Archimedean point from which Rawls' public reason can proceed. But that's because there are no Archimedean points. Period. Christianity is not an Archimedean point, either, nor does it proceed from one. It is one of many complex traditions of thought and feeling that give order and direction to the beliefs, emotions, attitudes, and behaviors of people who live within it or under its sway. Islam is another such tradition. Buddhism another. In the last century or two, non-religious traditions that have a similar effect have begun to develop -- enough so that some people with what Christians would recognize as a well-developed moral sense do not identify with religion at all and don't live too much under it's sway. (I'm not one of them, but I know a couple. OK, I know one.)
None of these traditions are moored to some absolute truth that is independent of and transcends mere human thought. I have no all-encompassing argument for or against religion generally or Christianity in particular. Insofar as it has good effects (and it does), I value it. Insofar as it has bad effects (and it does), I disapprove of it. But it is willfully naive to speak or act as though Christianity is (or proceeds from) some fixed point of truth that transcends human belief and disagreement. And to criticize Rawls for seeming to think "public reason" proceeds from such a point, while talking as though Christianity does proceed from such a point is a little silly. It's just another version of the mote/beam story.