Upon reading Godless: How an Evangelical Preacher Became One of America's Leading Atheists, I came across Dan Barker's excellent job of considering what it means to be an atheist and an agnostic. Referencing that Thomas Huxley coined the word agnostic by attaching a prefix a- (not, without) to gnostic, which is from the Greek gnosis (knowledge). The entire book is very insightful (particularly pages 96-98) and a worthwhile read for all (agnostic, atheist, and theist).