Mark Shea explains what he means by Christianist. I said I was done with Mark, but this was waiting for me in my stuff this morning, and I thought it was a good summation of Mark's thinking.
Basically a Christianist, according to Mark, is one who resists elevating a uniquely liberal narrative and political agenda to holy dogma, while reducing intrinsic evils and sins that cry out to heaven for vengeance to mere opinions over which we can lovingly agree to disagree. Note there is nothing at all about a person who would support abortion or gay sex or someone who would destroy an innocent man for political gain (or in order to maintain abortion) or someone who would reject key dogmas about the Faith yet call himself a Christian or Catholic.
Nope. A Christianist is a term Mark reserves for anyone who stands on the historical Christian Faith and disagrees with Mark on any one of a hundred topics regarding the Church's obvious slide toward acquiescing to the modern Left. It's Mark essentially saying how Christianists are the stereotypes of non-liberals as explained by Left wing media outlets, and whose motives are obviously evil, and whose hearts and minds are obviously full of sin.
In typical Mark form, anyone and everyone he chooses can be thrown under that label as the need arises. More than once, Mark has accused someone of being a 'Christianist', only to have that person object that they support the Church on almost everything he's said, but simply questioned a single issue. Which, for Mark, is good enough for the accusation of 'Christianist!'.
In short, it's the type of thing a stereotypical fundamentalist would do to cut off debate and insist that his way and God's way are obviously the same, so no sense engaging. Anyone who disagrees clearly 'doesn't believe the Bible is the Word of God.' An excellent conversation stopper in Protestant circles.
Again, I'm through with Mark, but figured this was a nice reminder why. You might as well go to a radical atheist site that explains an evil freak religious zealot is anyone who isn't an atheist. No sense wasting time. Unless it's something earth shattering, don't bother sending me anymore things from his pages. And if you must know why, you can't do much better than the link above.
UPDATE: to illustrate the point of Christianist being Markspeak for anyone Mark doesn't like who disagrees with Mark (sent by a kind soul who did the dirty work of linking to his site and copying the banality so I didn't have to):