Reading posts like this or this by Andrew Sullivan tend to make his statements concerning "Christianism" risible.
It is obvious from the context that he considers any Christian who actually believes the Bible to be a "Christianist," one who perverts Christianity into his own political philosophy, whereas authentic Christianity, in Sullivan's mind, is apparently that which okays whatever his "personal experiences" tell him is okay.
I am less bothered by his "buffet approach" to Christian doctrine than by his constant desire to demonize those as "Christianists" who do not take his approach (or at least who try not to, we are all I think guilty of ignoring passages that put demands on us that we do not like).
This also relates to the term "Islamist," which one in the end could define as "a Muslim who actually believes in a straightforward interpretation of Islam."
The overall goal here seems to be to deny to each faith the truth of what it is, to warp the belief system into something that makes secualr moderns comfortable, and then to demonize those who have not seclarized into something other than true adherents of the creed that they follow.
That is all.