"Christianist" is a strictly neutral term - it describes a specific political position about the relationship between Christian faith and the state. If I actually believed that Christianity is the one true religion, and that the US government should be based on my understanding of the dictates of Christianity, I'd think that Christianist would correctly describe me, and I wouldn't take offense. If you had said something like "evil Christianists," then I'd take offense.
I knew I'd seen this logic somewhere before. Where was it? Ah, yes:
II. If, upon these grounds, the adherents of the Church of Rome should not be called Catholics, or even Roman Catholics, by what name should they be designated?
To this we have no hesitation in saying that they should be called Papists or Romanists. They are accustomed to represent these designations as nicknames, and to resent the use of them as insulting. We do not regard them as nicknames, and we do not intend by using them to insult or wound the feelings of those to whom we apply them. We would not unnecessarily do anything that might be regarded as discourteous, and we would willingly concede to them any designation they choose to assume, provided it was in itself accurately descriptive, and provided it did not countenance any unwarrantable assumption on their part, or any unwarrantable concession on ours. Now, we know of no single or compendious designation for the adherents of the Church of Rome that answers these conditions, except Papist or Romanist. If they would suggest to us any other designations that answered these conditions, and were more agreeable to their feelings, we would willingly adopt them. But, in the meantime, we really do not know any others which we could warrantably or safely employ. Papist was the name usually given to them by the Reformers, and it has been generally used by Protestants ever since the Reformation, while it is only in modern times that those to whom it was applied have affected to regard it as a nickname, or to treat the use of it as insulting. It would be unfair and discourteous to give to them any designation which implied an ascription to them of anything which they disclaimed as erroneous and discreditable. But no objection can be adduced upon this ground against the names Romanist or Papist. . . . 
It's quite one thing to coin a term to describe those with whom you disagree--something which is frequently a matter of course among even the most gentle-minded of disputants--and quite another to expect those same people to regard your term in a friendly light. Let's not be disingenous, shall we?
 "On the Use of the Names 'Popery' and 'Romanism,' and 'Papist' and 'Romanist,'" The Bulwark or Reformation Journal 1 (August 1851): 24.