I think the roots of this term lie in 19th century europe at the start of Marxism.
Marxism was the ideology of the poor working class and therefore positioned against the rich bourgeoisie which happened to be very "religious" people. Also the profanity of some religious leaders and the luxury of the catholic church (otherwise referred to as religious Art

) were a point to attack for the early Marxists. Another strong issue was the use of church and religion to oppress the lower classes (first addressed during the reformation in the 16th century).
This is why the "left" (speak: Marxist) ideology chose to turn away from chruch and (because in simplifying this is often attributed as equal) from religion.
This weird arrangement stuck with many left-wingers until today, but what also can be observed is that a lot of left-wingers who despise the Catholic church are very open minded to some other religions (be it reformed Christians or far-east religions) though also opposing some (namely Islam) for being very authoritarian.
The contradiction between "The Left" and the Catholic Church is also founded on the conservative stance of the CC on several issues (abortion, contraceptives, same-sex marriage, ...)
Especially in countries where the distinction between conservatives and progressives seems to be clear,religious (Christian) people if taking a political career are often driven into the conservative parties not so much because of policy but more because of peer-pressure inside the party.
So, while for a normal person it may be completely okay to have "left" political beliefs and be a Christian sticking to the Bible, for a politician this is not that easy.
The definition of the "religious right" often drives me crazy myself:
Although an agnostic I was raised with Christian beliefs and do still hold many of them dear. But even a believing Christian can sometimes find himself in trouble with whom to associate on e.g. the following issues:
+ Death Penalty
+ Abortion
While a true Christian IMHO should oppose both it is commonly displayed in public that there are only two sides on these issues:
+ "The Religious Right": ProLife (=Anti-Abortion) and pro Death Penalty
+ "The Atheist Left": ProChoice (=Pro-Abortion) and against DP
Strange isn't it
[ 10-25-2003, 07:36 PM: Message edited by: Faceman ]