But that's their problem, believing something I feel is untrue.  

I'm not going to go call them illogical followers of a mishmash of creeds drawn from all conceivable sources who fail to realize or even acknowledge that fact, despite the fact that it is often true.  (And to Christians who actually study your faith and intellectually engage with it, ignore what I have said up to here, I have absolutely no beef with you - we may disagree on truth, but I admire you for trying.)

But when the word "Christian" itself becomes a pejorative term, the Phelpses and Falwells of this world have succeeded, and they did so because real, moderate, Christians failed to seperate themselves from the insane ones.  I won't use the term Christian as an insult.  And many of the Phelps counter-protesters did and do, which is a problem, even for someone as agnostic and aspiritual/amystical as I am.  

I don't know how far away you are from campus political life, but I lived on a fairly political campus, for 12 years growing up and then another 4 and a half while attending classes there, and have only been gone from it for about a year and a half.  Being a nationally famous university, we attract a lot of attention from a lot of completely insane people - Phelps visits regularly, the KKK used to, there are organized Maoist and Communist movements on campus (let me distinguish - Soviet-style Communism, an insane idea, as opposed to the general idea of Communism).  Whether or not you want to be involved in the political discourse, you can't avoid it - it's thrust into classes, it engages you as you walk by it occuring on campus, it is unavoidable.  And wherever I went, I encountered the problem of people using a description of belief as a term of insult - using Christian, or Jew, or Socialist, or Communist, or Capitalist, or Muslim, as terms of insult.  

There are two problems - that of divorcing a descriptor from the things it describes, and that of conflating different things under the same descriptor.  They are essentially two different ways of viewing the same problem.  Divorce of the descriptor from the described allows an easier conflation, in that it tends to lead people not to look at what lies behind a term.  It's a lot easier to hang a blood libel on a Jew when the term "Jew" has already become divorced from Judaism in the head of the listener.  It's a lot easier to hang an association of "terrorist" on a Muslim when the listener doesn't know anything about Islam.  The related problem, that of conflation, is another way of divorcing the descriptor from meaning, this time by expanding the range of the meaning to the point where it becomes so non-specific as to be useless.  Christian is getting to this point, considering the huge gamut of views that all associate themselves with the term - everyone from Catholics to Seventh Day Adventists to Greek Orthodox to snake-handling cults to Phelpsian crazies is a Christian, so what meaning does the term even have?  The only way to recover from massive conflation is to reduce the meaning of the term - Christian, at this point, essentially implies "I believe Christ was the messiah" and that's about it.  The best defense to prevent conflation is what I am requesting - defending the meaning of the descriptive terms before they can be corrupted.

A similar situation I run into all the time is with Messianic Jews who don't understand my objection to them calling themself Jewish.  As a Jew, I don't wish to be associated with people who believe Jesus was the messiah, as messianics do.  I believe that the terms need to be seperated, so that it is very clear that people realize that my brand of Judaism (the tradition of Rabbinic Judaism, not a specific branch of it) does not acknowledge a messiah, and that Messianics have no real right to a claim on that term.  It's a relatively restrained example, in that no one is likely to get into a fistfight over it, but it's the same situation.  If you don't point out the differences between you and someone who wants to call themselves by your name, you've lost any reason to believe a bystander will know those differences, or even should.