People in Australia are completely different to Americans as far as advertising their faith. I may see the odd 'fish' symbol on a car but generally christianity here is a cult.
Usually I see a cult defined as some group that "rejects the biblical teaching that salvation is a “free gift” from God by grace through faith alone. Instead, cults and all other “religions” falsely say that people must “earn” their salvation by certain works and deeds, often works that will perpetuate the cult (this glorifies man rather than God)."
And "cults are often exclusive, centering around a strong leader (or late founder) who has the only “true” interpretations of Scripture. Whereas Christ came to give us freedom, (Galatians 5:1), cult members are in bondage to that cult leader, with the threat that leaving the cult will cause a loss of salvation.
So I don't see how Christianity can be defined as a cult unless it's some denomination or church that claims to be Christian but really isn't.