The Roman Catholic Church was one of the victims of the French Revolution. Many of the priests, nuns, and other clergyman were abolished. Why did the revolutionaries "hate" the Roman Catholic Church?
One might argue that secularism brought about the French Revolution; not all correlation is causation.
Mike Duncan's Revolutions podcast covers the role of the church in the revolution fairly well. (at least at a high/brief level).
There are a couple of factors worth mentioning.
The French Revolution wasn't designed; it was a runaway cart, heading down a hill spilling ideals at every bump. So any conclusion/answer that doesn't take into account the ever changing goals/demographics/constituencies/random chance is incomplete. Look at the number of leaders of the revolution that were consumed by/beheaded by the revolution. The Church was inevitably going to get caught up in the garbage disposal of revolution. Nobody was innocent, everyone was to be attacked.
The French Revolution arose from what we would now call a failed state. The ancien regime was unable to exert executive power in any effective fashion. All institutions had failed; all institutions were on the table. Although the Church had somewhat effective institutions at the street level, the Church's legitimacy never recovered from the investiture scandal. The legitimacy of the church derived from divine assertion, which was difficult to replicate, prove, or discuss. The Church hierarchy was neither engaged with, nor flexible enough to adapt to the new Revolutionary government. (I'd argue that the legitimacy of the French Revolution was based on a similar revelation; the only discussion permitted was via cannon and guillotine. The two powers were intrinsically going to come into conflict).
@Himarm's comment above is a pretty good summary. I'm going to include his comment below to avoid link rot, but all credit should go to @Himarm
the monarchy/nobles/clergy were typically the 3 power structures in Europe. The church was partially corrupt like the nobles king ect, and typically the church would side against the commoners, even though the commoners felt that the church should side with them. So since the french revolution was essentially, destroy everything, start over. they destroyed everything. Unlike the American revolution, that was specifically aimed towards British interference.
The critical period of the National Assembly (the episode I linked in the prior paragraph) was a bizarre case of everyone proposing that someone else's prerogatives be nullified. The National Assembly fought for some time to find a way to preserve the relationship with Rome. As I understand it, they thought they had a solution. THey were, unfortunately, mistaken.
The French Revolution took place in the context of a global revolution in epistemology & belief. The power of the church had been on a decline, the power of science had been on the rise. This was going to affect the revolution somehow.
Although the National Assembly included clerics, many of them were Bishops and above who were not aligned with or incentivized by Rome. Counterintuitive, but true.
I could probably cite a dozen more reasons, but none of them will answer your question, because your question presumes that the two events were related in a simple fashion, and reality was far more complex.
Freemasonry's Role in the French Revolution
The role of Freemasonry both outside the Catholic Church and as it infiltrated into the Church before and during the French Revolution is pivotal. Eight popes between 1738 and 1890 have vehemently and explicitly condemned Freemasonry and its naturalistic¹ ideology; this is more than the Church has ever condemned any heresy in its entire history.²
Voltaire, Rousseau, Diderot, and other atheist (i.e., secular) philosophes, coupled with the press, were the mouthpieces of the Freemasonic and thus anti-Catholic ideology.
The Philosophes' anti-Catholic Writings
For example, Diderot wrote:⁴
Et des boyaux du dernier prêtre
Serrons le cou du dernier roi.
[With the guts of the last priest
let us strangle the last king.]
I am tired of hearing it said that twelve men sufficed to establish Christianity, and I desire to show that it requires but one man to pull it down.
When hearing about the suppression of the Jesuits, he said:
See, one head of the hydra has fallen. I lift my eyes to heaven and cry "crush the wretch."
I finish all my letters by saying "Ecrasons l'infâme, écrasez l'infâme" ["Let us crush the wretch, crush the wretch"], as Cato used one time to say, "Delenda est Carthago [Carthage must be destroyed]."
He wrote to Damilaville:
We embrace the philosophers, and we beseech them to inspire for the wretch all the horror which they can. Let us fall upon the wretch ably. That which most concerns me is the propagation of the faith of truth, and the making of the wretch vile, Delenda est Carthago.
none of his works attracted so much attention as his "Mémoires pour servir à l'histoire du Jacobinisme" (London, 1797-98). It appeared in an English dress: "Memoirs of the History of Jacobinism and Freemasonry of Barruel, translated into English by the Hon. Robert Clifford" (London, 1798) in four volumes. This important work is an endeavor to account for the French Revolution by a study of the anti-Christian and anti-social principles of the secret societies and the encyclopedic philosophers. Owing to its translation into every modern language it was everywhere read and commented upon. A sharp criticism in the "Monthly Review" brought forth a reply from Barruel who greatly increased the circulation of his book by issuing an abridgement of it in 1798. The Freemasons of France, Germany, and England angrily contested his assertions and a voluminous literature was the consequence. While some are of the opinion that Barruel's work attributes to the secret societies many evil deeds for which they are not responsible, all admit that his exposition of their principals and the logical consequences flowing from them is the work of a powerful mind. Barruel, indeed, seems to have been the first to portray clearly the necessary consequences to civil government, to the Church, and to the social order that must result from the atheistic oathbound associations that had acquired such tremendous power on the continent of Europe.⁷
1. Pope Leo XIII's 1884 encyclical Humanum Genus, subtitled "On Freemasonry & Naturalism."
2. Gruber, H. (1910). Masonry (Freemasonry). In The Catholic Encyclopedia. New York: Robert Appleton Company.
3. Dillon, George F. Grand Orient Freemasonry Unmasked. London, 1885.
4. attributed to Diderot by Jean-François de La Harpe in Cours de Littérature Ancienne et Moderne (1840); cf. this
5. All the following Voltaire quotes come from Grand Orient Freemasonry Unmasked ch. III. "Voltaire," pp. 9-10.
6. original French title: Mémoires pour servir à l'histoire du Jacobinisme
7. Fanning, W. (1907). Augustin Barruel. In The Catholic Encyclopedia. New York: Robert Appleton Company.
8. cf. also: Goyau, G. (1912). French Revolution. In The Catholic Encyclopedia. New York: Robert Appleton Company.
One ought to remember that the Church and clergy were the FIRST of France's three "Estates." The nobility was "only" second. The third was the people.
In the French Revolution, the Third Estate (people) rose up against the authority of the other two. Although the most dramatic images include the execution of the king and queen, the Church was "displaced" also, leading to a power vacuum that "secularism" filled.