to me, a Christianist is a person who has stopped following Christ and has started following Christianity as a movement, a source of guidance in all areas of life but especially political decision-making and activism. in a nutshell.
to Andrew Sullivan, Christianism is much more personally offensive. an excerpt from his 2006 Time Magazine article entitled, My Problem with Christianists:
So let me suggest that we take back the word Christian while giving the religious right a new adjective: Christianist. Christianity, in this view, is simply a faith. Christianism is an ideology, politics, an ism. The distinction between Christian and Christianist echoes the distinction we make between Muslim and Islamist. Muslims are those who follow Islam. Islamists are those who want to wield Islam as a political force and conflate state and mosque... I mean merely by the term Christianist the view that religious faith is so important that it must also have a precise political agenda. It is the belief that religion dictates politics and that politics should dictate the laws for everyone, Christian and non-Christian alike.
That's what I dissent from, and I dissent from it as a Christian. I dissent from the political pollution of sincere, personal faith. I dissent most strongly from the attempt to argue that one party represents God and that the other doesn't. I dissent from having my faith co-opted and wielded by people whose politics I do not share and whose intolerance I abhor. The word Christian belongs to no political party. It's time the quiet majority of believers took it back.