In this visceral letter to his religious brother, we see the social origin of Diderot's militant Atheism. Diderot’s movement toward atheistic materialism was shaped not only by intellectual currents of his time, but also by the more intimate tensions of his own family life. This letter, written with remarkable restraint yet unmistakable frustration, reveals a portrait of strained familial relations in which religious conviction—particularly that of his younger brother, the canon of Langres—serves less as a guiding light than as a corrosive force. We begin to see that Diderot’s philosophical orientation was not merely the outcome of speculative reason, but deeply entangled in the dynamics of a family split between piety and irreverence, obedience and critique. What ignites Diderot’s fury is his brother’s abandonment of what he believes is familiar responsibility- particularly to their brother. The charge is not belief but violence in belief’s name, emotional estrangement enacted as a form of righteousness. “If your beliefs permit you to hate me,” he writes, “why should mine not permit me to hate you?” In his edition of Diderot's unpublished works, Naigeon positioned this letter by Denis Diderot immediately after the Apology of Abbé de Prades, a placement justified by thematic and philosophical connections. Much of the content from Diderot’s letter would later reappear, often in adapted form, in the Encyclopedia article titled “Intolerance.” One figure relevant to this context is Abbé Caveirac, author of the Apology of Louis XIV and His Council on Revoking the Edict of Nantes (1758). Caveirac was convicted in 1764, sentenced to the pillory and exile for his defense of the Jesuits. Notably, he had also engaged in a public debate with Rousseau over musical theory, reflecting the broader intersections of theology, politics, and aesthetics in this period. This modern edition contains a new Epilogue by the translator, a glossary of Philosophical Terms used by Diderot, a chronology of his core life and works, and a summary index of all of Diderot's works. With a clean, modern translation of Diderot's Enlightenment-era French, this edition brings Diderot's thoughts directly into the modern intellectual sphere, tracing the intellectual forces which swept along Diderot and impacted today's secular world. This conflict with his brother was not isolated. Diderot’s whole intellectual project—his materialism, his rejection of immaterial souls, his critiques of religious dogma in the Encyclopédie and elsewhere—can be read as a reaction against precisely the kind of ecclesiastical logic embodied in his own family. The younger brother, as canon of Langres, personified the institutional power of the Church, but also its blindness to interiority and individual conscience. Diderot’s insistence that “conscience must be enlightened, not constrained” becomes intelligible as more than abstract theory—it was a personal plea. The rejection he experienced within his family made Christianity not just intellectually indefensible, but emotionally suspect. His eventual atheism was not merely negation, but the search for a moral vocabulary untouched by clerical coercion. Diderot’s embrace of materialist philosophy and anti-religious critique was not simply rebellious. It was protective. The Church had entered his home, set sibling against sibling, and claimed moral authority over love, conversation, and thought itself. In becoming an atheist, Diderot was not only rejecting the metaphysical doctrines of Christianity; he was defending the legitimacy of familial love unconstrained by dogma. His later writings, filled with explorations of corporeality, moral sentiment, and natural ethics, are often read through the lens of Enlightenment rationalism. But at their core is also the memory of a brother’s condemnation, a rupture that turned belief into a boundary, and drove Diderot toward a philosophy that could promise—if not comfort—at least consistency between thought and affection.