When I was growing up in the fifties and sixties, almost no one in politics or everyday life went around proclaiming, “I am a Christian.” If indeed you were a Christian—that is, someone who considers Jesus Christ the Messiah—you identified yourself as a Lutheran, a Methodist, a Baptist, a Catholic, and so on in excelsis in order to let others know where you stood in the vast American religious landscape.
Calling oneself a Christian today, by contrast, has a special, politicized meaning. For most people in public life, this self-identification suggests a particular form of conservative Christianity, a brand of religion that seeks not only to proselytize but to impose its values on others through the machinery of the state. The huge exception to this rule is President Barack Obama, who has been forced by the birther-paranoids to advertise his credentials as a Christian in order to refute the lie that he is a “secret Muslim.”
Once upon a time (until around 1980, actually), the appellation “Christian” used to mean “right-wing Protestant,” as a consequence of the historic animosity between many forms of American Protestantism and the Roman Catholic Church. That is no longer true, as demonstrated by GOP primary hopefuls Rick Santorum and Newt Gingrich, the darlings of Protestant fundamentalists, although they personify the cliché “more Catholic than the pope.” (In Gingrich’s case, the relevant pontiffs would be certain medieval and Renaissance vicars of Christ who produced numerous children through extra-pontifical liaisons.) Santorum is in fact a Catholic fundamentalist—unlike the majority of American Catholics, who do not accept either the notion of papal infallibility or the Vatican line on sexual behavior. Liberal Catholics, well aware of the political meaning of Christian in American politics, generally call themselves plain old “Catholics.”