I think this is a brilliant essay. It is very beautiful. I also think you are fundamentally right.
1) your belief that Christianity implies social solidarity and help for the least among us is different from their belief that Ayn Rand was a Christian. Your interpretation is based on the text. But if their goals are secular, so are yours. Its none of my business (I'm an atheist) but it seems to me that Erick Erickson isn't a Christian at all. So it seems to me that his view that you aren't one is dead wrong, but not at all in the way you think he is dead wrong. Now as a practical matter, I think that one should never tell someone who thinks he is a Christian that he doesn't know what the word means. It is rude and leads only to bad consequences. That typed, there is no doubt in my mind that Erick Erickson doesn't know what the word "Christian" means (note I trust he won't read this comment [blog] thread).
2) what does "validity" as in "validity of other religions" mean ? To me a set of claims are valid if they are true *and* we have good reason to believe they are true.. Believing in the validity of two different religions seems to me to be logically impossible. I think the limit of sane toleration is to believe that believers in different religions haven't made a mistake -- an intellectual error. I suppose I would accept "equal validity." I don't think any religious beliefs are valid (I'm an atheist). But I don't think atheism is valid -- I have no proof that there is no God and no doubt that there is no God. I think I can manage to be tolerant (and even at times polite) without accepting the validity of your faith. I think that the issue of religion causes liberals to abandon language and logic. In any case, I ask for your definition of "validity" in that context.
3) Is the key role of religious freedom in promoting religion really forgotten ? The argument was made (also eloquently) by Mill in "On Liberty" . Also by Walter Mondale -- OK you may have a point there -- "aid by Mondale" and "ignored" are closely related concepts. But surely people have noticed the fact that the US separation of church and state was extraordinary and almost incredible at the time (and for almost a century, somewhere Marx, for example, noted the astounding fact that in some states Jews could be elected and didn't grasp the incredible fact that this was true in all states) and the other fact that the US is much more religious than other rich countries. But yes you are right -- it is almost impossible to believe that people could be so blind that the fail to notice the two facts, but people manage.