This post I made in another thread explains the basics.
In fact you might wnat to read the OP in that thread because it goes hand in hand with the main header question.
What exactly happen that made French culture as a whole apathetic to maintaining a family line to keep the Catholic tradition as opposed to Italians, Irish, Poles, Spanish speaking nations, and southern Germans? I mean in the above cultures even merely converting to another sect of Christianity or even a Catholic offshoot that disproves Papal authority such as the Old Catholic can warrant immediately disownment from the family. Even in America among Catholics who can trace their family from the time of the American Revolution, they have a bit of trouble with relatives converting to other Christian faiths. Not outright outcast from the family as say a typical traditional Mexican family would but definite disproval towards any child interested in Methodism or other sects.
Where as the French culture today are so indifferent about conversion to other faiths. So long as its a distinctly Western mainstream Christian religion such as Lutheranism and the Church of England I noted in my thread most French Catholics-including first generation immigrants to America- don't really care. Even my experience in Quebec seems to confirm French indifference towards religion.
Why is this? I mean weren't the French quite fanatical in the medieval ages with their defense of Christendom during the Dark Ages from Muslim armies and Viking raiders, persecution of Huguenots, sending the most volunteers in the Crusades, and frequent participation in the European Wars of Religion?
This is a very complicated question but certainly critical factors include the close institutional alliance between the Roman Catholic Church and the French monarchy, a perception on the part of a cross-section of French society that the Church had become corrupt and enormously wealthy through its historical involvement in French politics, its enjoyment of numerous privileges the commons did not, and the birth of the Enlightenment which irrevocably changed French political philosophy, theories of government, and the role of religion within the state. All of these historical factors contributed to the beginning of the French Revolution which can be seen as a massively important point of rupture that completely repatterned the relationship between the Church and state (and indeed much else).
The taxation system of the Ancien Regime was deeply flawed in the sense that the two wealthiest estates (the clergy and the nobility) did not pay taxes, leaving the entire burden to be serviced by the poor and middle class. Tax farmers preyed upon the most vulnerable segments of the populace for what profits they could find, and tax evasion was rampant. Food shortages were further aggravated by significant natural disasters such as hailstorms, which destroyed French crops leading to large-scale starvation and riots. People were cognizant of the fact that piety and the prayers of the clergy did nothing to abate the scale of these disasters and the lived experience of ordinary French citizens was appalling. Meanwhile the clergy continued to present natural disasters as evidence of divine judgment, meted out upon the French for the manifest sins. (Nigel Aston, Art and Religion in Eighteenth-Century Europe, 44)
While the people struggled to feed themselves and their families the Church and its extraordinary wealth became associated with an inept and callous regime that did far too little to intervene with effective policy reforms and charitable giving.
With the beginning of the French Revolution its intellectual and political leaders embarked upon a systematic process of "dechristianization" which sought to strip the Church of its privileges, wealth and position of authority within society. It was accused by Robespierre and his supporters of being a bastion of superstition that served to keep the peasantry servile and subordinate to the whims of a divinely chosen monarch. In effect it was seen as an instrument of power whose spiritual role was a disingenuous cover for its real ambitions which were control and influence. At the village level Roger Chartier has noted that the common people were fiercely critical of the lower clergy, their paucity of numbers, unavailability for religious services, and their intellectual mediocrity due to lack of education. (Chartier, The Cultural Origins of the French Revolution, 146)
This line of radical thought was symptomatic of an aggressively reformist political climate that sought the total secularization of French society through a commitment to classical liberal values that necessitated the protection of personal freedoms. Political clubs and theatres constantly produced depictions of clergy that indicted their morals and conjured up the worst episodes in Catholic history. (Susan Maslan, Revolutionary Acts: Theater, Democracy, and the French Revolution, 41) Favourite subjects included the St. Bartholomew's day massacre and the Inquisition. A civic religion committed to the interests of the state would be acceptable but not an institution identified with the most reactionary aspects of the Ancien Regime, not to mention those foreign autocratic powers that sought to destroy the Revolution before it could spread.
The degree of often violent dechristianization in France during this period was truly extraordinary and its imprint remains on the fabric of French society to this day. The liberal values of liberty, equality, and fraternity would become the basis for secular French culture, law and government, all of which are meant to be based on reason and rationalism unencumbered by traditional religion or aristocratic privilege.
Susan Maslan, Revolutionary Acts: Theater, Democracy, and the French Revolution
Christopher Hibbert, The French Revolution
Nigel Aston, Art and Religion in Eighteenth-Century Europe
Chartier, The Cultural Origins of the French Revolution