Historicly you are right YB, but you are thinking like an american, french are not like that, they change very fast, maybe too fast sometime… France was really catholic not so long ago, but it is not the case now. We have no strong Catholic/Christian party, and if the stat said only 12% catholic it is becaue of an exageration of Agnostic, certainly due to the question… At the limit, an agnostic can be consider someone that is not sure. But as always i will argue your Statistics is invalid because there is no survey, and that is for that reason Germany get so much “Unnafiliated”, not because the communist convert a lot of people, (sure they did but not that much) but because the statistic only consider how people are born, i can assure you christianism is stronger in Germany than in France.
I am sure that i you had visit france you would understand how religion is weak. Church are never full, priest are getting really old and our politician never finish their speach with “que dieu bénisse la France”. As CC said, the “old world” start to let down tradition in general, after the WWII there was a renew in Europe.