1) One who adheres strictly to any set of basic ideas or principles.
2) Members of an American Protestant movement of the early 1900's that was based on a 12-volume series of books called "The Fundamentals." The Fundamentals was a scolarly work compiled from the contributions of various Christian scholars, and given away free to pastors, missionaries and others. The purpose of The Fundamentals was to layout the most basic tenets of Christianity, without which it would cease to be Christianity. Some of these tenets include the Diety of Christ, the virgin birth and the resurrection.
3) An intended slur hurled by angry atheists.
Mormons are not fundamentalists because they do not believe that Jesus was the only son of God, who came to die to pay for the sins of everyone who trusts in him.