Denis Diderot (1713-1784), born in Langres, France, was an influential essayist, novelist, philosopher, and promulgator of Enlightenment values, known for his willingness to position himself in opposition to established authority.
Diderot was an atheist materialist who promoted reason over and above hidden knowledge of religion and religious practices such as the mystical or occult. Diderot’s book The Skeptic’s Walk (completed in 1747, published in 1830) conveys his transition to atheism through the use of a fictional story. It includes a fictional dialogue between an atheist, deist, and a pantheist, and is critical of Christianity, an Abrahamic concept of God, as well as the Church.
Diderot is most famous for managing to get some of France’s leading intellectuals, including philosophers, writers, and scientists, to write articles for the Encyclopedia, or Reasoned Dictionary of the Arts, Sciences, and Trades (1772). This text, often referred to in its brief form Encyclopédie, took 21 years to complete and consists in its final form of no less than 17 volumes of texts and 11 volumes of illustrations. Diderot was both the editor and contributor to this massive work.
The Encyclopédie’s most striking feature was its critical approach to contemporary ideas and institutions. Its contributors were advocates of both secular and scientific thought who applied reason and logic to explain human experience and phenomena of the natural world. They did so without making use of religious and political beliefs, dogmas, and ideologies as explanations. Their work proved a challenge to the religious and aristocracy, in particular the Catholic Church and the French monarchy, both of whom saw their authority based upon divine approval and traditional ideas of an unchanging order.
As such, Diderot and the authors of the Encyclopédie fell well within the values of the Enlightenment, an intellectual movement that begun in the early 18th century, which based its ideas upon scientific and philosophical thought arising from the 17th century. Diderot is not the only name associated with this movement as the likes of Montesquieu (1689-1755), Jean-Jaques Rousseau (1712-1788), and Voltaire (1694-1788) also produced works of their own, as well as contributed to the Encyclopédie. Although these works were broad and ranged across numerous topics, there is an agreement on three major areas:  a desire to base society on rational thought as opposed to religious faith and the doctrines of the Catholic Church,  the emphasis on observation and experimentation in science, and  pursuing ways to organize society and the state based upon natural law and justice.
Diderot divided the Encyclopédie into three sections: memory (which makes connections to history), reason (philosophy), and imagination (poetry). What is omitted is any mention or reference to God or the divine, and theology and religion was simply viewed as part of philosophy. This was quite revolutionary given that belief in God and the divine had been central to the lives of Europeans for centuries, but its privileged position was denied in the Encyclopédie. There were attempts to oppose the Encyclopédie, censure its contents, and threaten its authors, which included Diderot’s own arrest and incarnation. There were also searches for manuscripts of the Encyclopédie the authority’s believed he had hid in his home.
The repression and controversy, however, did not stop Diderot’s ideas becoming hugely influential. His ideas are still alive today, and had even influenced revolutions later in the France and the United States at the end of the 18th century.