A few points here:
1) "Defining" something is not the same thing as giving an "exhaustive definition" of something. One can arrive at a "workable definition" of something that is not necessarily "exhaustive". In fact, some things can never be comprehensively defined. That doesn't mean they can't be defined at all.
2) If you are arguing against Christianity using "the basics of the Bible" then you have already contributed to the "definition" of the word -- that is, you have implicitly defined it as someone who feels a certain way about the Bible -- a way that may or may not be supported by many who call themselves Christians.
3) "Defining" Christianity or giving a taxonomy of Christian sects could be a rewarding activity on many levels whether or not Christianity is true.
***** Orion Auld ***** *------------------------------------------------* "We are only fabulous | "The mind boggles. It even plays scrabble." | beasts, after all." | -- christopher g. williams: cgw@io.com | -- John Ashbery *------------------------------------------------*