The usage is noted in a William Safire "On Language" column on May 15, 2005:
"Two weeks after writing about the fervor of the late Terri Schiavo's ''Christianist 'supporters,''' Hendrik Hertzberg of The New Yorker last month described Representative Tom Delay as a ''hard-right Christianist crusader.'' A few months before, soon after President Bush was re-elected, the conservative Weekly Standard reported that an Ohio cartoonist had sent out a communication deploring ''militant Christianist Republicans.''
So apparently, you can use the term as you see fit.... ;)