Roman Catholicism is the largest denomination of christians in the world, but they are not the only christians. Besides the Eastern christian churches you have the many denominations that followed the Protestant Reformation.
You will get many answers that are bigoted against either the Roman Catholic Church or some other denomination. Those that proclaim they know what a "true christian" is and they will provide some passage to "prove" their point. You can ignore them as the bigots that they are. People have different interpretations of the bible, and have different churches that reflect their beliefs.
Frankly, I think they are all wrong, the bible is a work of fiction, and many passages I would consider immoral. However I hate bigots that want to tell people that their brand of crazy is better than others.
Go ahead and pick out the bigots, read the answers from James and Keith for example.