The ACLU has stated officially that if the government ever tried to squelch Christian Protestant fundamentalism, it would stand behind the free-speech rights of Christian Protestant fundamentalists.
The ACLU repeatedly stood behind the free-speech (and other) rights of Jehovah's Witnesses, and from those cases many of the Bill of Rights were incorporated through the 14th amendment to apply against the states. This finally gave the 14th Amendment some of the force envisioned by its Radical Republican drafters.
So actually, we already have the ACLU joining forces with Christian fundamentalists to thank for quite a few of our rights.
