First used by Thomas Henry Huxley in a review of Charles Darwin's The Origin of Species in the April 1860 issue of Westminster Review, it has since become a term used by creationists to poison the well when discussing (attempting to discredit) the theory of evolution. By adding the -ism, they imply a system of beliefs or an ideology, hypocritically equating Darwinism to any other crackpot idea.
Calling someone a "Darwinist" or "evolutionist" is about as ridiculous as calling someone who accepts the fact of gravity a "gravitationalist," or someone who believes the earth is spherical a "sphericist."
Darwinism is wrongly thought by many fundamentalists to be the "religion" of atheists. It's true that most atheists accept Darwin's theory, but that fact is irrelevant to their nonbelief in a deity. They simply see no reason to reject something that's obviously true (i.e., it doesn't contradict some other belief that they refuse to give up).
Darwinism isn't a belief system or an ideology. A person who accepts Darwin's theory (after 150+ years and mountains of evidence in its favor) is merely non-delusional.