I like the fact that the word "christianist" exists (even if I'm unlikely to use it with the precise meaning that Sullivan intends) because I think its important to differentiate between what Christianity means as expressed in the Gospels and what Christianity means as expressed in current public discourse.
One of the overriding themes of the New Testament is that of hypocrisy. Judge not, lest you be judged, when you pray, pray in secret, render unto Caesar that which is Caesar's, don't speak of the speck in your brother's eye when you have a log in your own..etc.
The degree to which politically active Christian sects stray from these ideals is the degree to which I want a label to separate their beliefs from my own.
Call me selfish but I hope that "christianist" makes Oxford's English within ten years.