On this day (7 May) in 1794, amid the infamous “Reign of Terror” phase of the French Revolution (La Terreur, September 1793 to July 1794), the National Convention of the First French Republic (République française, 1792–1804) officially replaces the Revolution’s atheistic “Cult of Reason” with a new state religion known as the “Cult of the Supreme Being” (Culte de l’Être suprême). Here is some background via Wikipedia (footnotes and some hyperlinks omitted):

“The French Revolution had caused many radical changes in France, but one of the most fundamental … was the official rejection of religion. The first new major organized school of thought emerged under the umbrella name of the Cult of Reason. Advocated by radicals like Jacques Hébert and Antoine-François Momoro, the Cult of Reason distilled a mixture of largely atheistic views into an anthropocentric philosophy. No gods at all were worshipped in the Cult of Reason; the guiding principle was devotion to the abstract concept of Reason.

“This rejection of all godhead appalled Maximilien Robespierre…. He thought that belief in a supreme being was important for social order, and he liked to quote Voltaire: ‘If God did not exist, it would be necessary to invent him’. To him, the Cult of Reason’s philosophical offenses were compounded by the ‘scandalous scenes’ and ‘wild masquerades’ attributed to its practice. In late 1793, Robespierre delivered a fiery denunciation of the Cult of Reason and of its proponents. and proceeded to give his own vision of proper Revolutionary religion. Devised almost entirely by Robespierre, the Cult of the Supreme Being was authorized by the National Convention on 7 May 1794 [18 Floréal Year II] as the civic religion of France.”

More details about this tumultuous chapter in French Revolution history are available here: Mathias Sonnleithner, “More Voltaire than Rousseau? Deism in the Revolutionary Cults of Reason and the Supreme Being,” in Anna Tomaszewska (editor), Between Secularization and Reform: Religion in the Enlightenment, Leiden, The Netherlands: Koninklijke Brill (2022), pp. 160-196. See also this short report by Rumaysa Haqqani (via The Collector): “When Robespierre Created the Cult of the Supreme Being“.