Andrew Sullivan explains why he uses the word "Christianism"

"Many evangelicals loathe my use of the word Christianism, rather than Christianity, to describe the fusion of political power and religion to police the moral lives of others. They recoil at the echo of Islamism - although we know many Islamist parties, as in Turkey or Indonesia, that do not engage in terrorism, but merely believe in the fusion of church and state as emphatically as older American evangelicals do. 

"That's why it's revealing to see a major figure in the American Christianist movement, Pastor David Dykes, - he opened the 2008 Congress with a prayer - openly advocate the execution of gays, and criminal penalties for exercise of free speech in defense of gays, in Uganda. If these people could, they would do the same here. The constitution protects us - especially our right to speech. But what these theocrats want is as undeniable as it is repellent in a free or humane society. 

"More to the point, here is an alleged Christian demanding that those on the margins of society not be embraced, as Jesus practised, but be executed, as Jesus' Roman executioners did. It doesn't get more anti-Christian than that. To even associate the word Christianity with these sentiments is an attack on Jesus Christ."

 -- Andrew Sullivan