The 1789 Declaration of the Rights of Man and of the Citizen, which is considered by legal authorities to have equal legal standing with the Constitution of France, states: "No one may be questioned about his opinions, [and the] same [for] religious [opinions], provided that their manifestation does not trouble the public order established by the law." and:
"The law has the right to ward [i.e., forbid] only actions [which are] harmful to the society. Any thing which is not warded [i.e., forbidden] by the law cannot be impeded, and no one can be constrained to do what it [i.e., the law] does not order."
Thus it follows that the French government cannot arbitrarily regulate and prohibit religious activity; it is strictly constrained to regulate it only to the extent that there is a need to safeguard public order and prohibit actions harmful to society (such as, for instance, human sacrifices).

http://en.wikipedia.org/wiki/Freedom_of_religion_in_France