Atheism, as defined by the entry in Diderot and d'Alembert's Encyclopédie, is "the opinion of those who deny the existence of a God in the world. The simple ignorance of God doesn't constitute atheism. To be charged with the odious title of atheism one must have the notion of God and reject it." In the period of the Enlightenment, avowed and open atheism was made possible by the advance of religious toleration, but was also far from encouraged.

Accusations of atheism were common, but most of the people suspected by their peers of atheism were not actually atheists. D'Holbach and Denis Diderot seem to be two of the very small number of publicly identified atheists in Europe during this period. Thomas Hobbes was widely viewed as an atheist for his materialist interpretation of scripture—Henry Hammond, a former friend, described him in a letter as a "Christian Atheist". David Hume was accused of atheism for his writings on the "natural history of religion";Hume on Religion (Stanford Encyclopedia of Philosophy) Pierre Bayle was accused of atheism for defending the possibility of an ethical atheist society in his Critical Dictionary, and Baruch Spinoza was frequently regarded as an atheist for his "pantheism". However, all three of these figures defended themselves against such accusations.

Rise of toleration In the Reformation and Counter-Reformation eras, Europe was a "persecuting society" which did not tolerate religious minorities or atheism.Ole Peter Grell and Roy Porter, "Toleration in Enlightenment and Europe" in Toleration in Enlightenment Europe, ed. Ole Peter Grell and Roy Porter (Cambridge: Cambridge University Press, 2000), 1. Even in France, where the Edict of Nantes had been issued in 1598, then revoked in 1685, there was very little support for religious toleration at the beginning of the eighteenth century.Marisa Linton, "Citizenship and Religious Toleration in France" in Toleration in Enlightenment Europe, ed. Ole Peter Grell and Roy Porter (Cambridge: Cambridge University Press, 2000), 157. States were concerned with maintaining religious uniformity for two reasons: first, they believed that their chosen confession was the way to God and other religions were heretical, and second, religious unity was necessary for social and political stability.Linton, 157–158. The advancement of toleration was the result of pragmatic political motives as well as the principles espoused by Enlightenment philosophes. Religion was a central topic of conversation during much of the eighteenth century. It was the subject of debate in the coffeehouses and debate societies of Enlightenment Europe, and a bone of contention among the philosophes. Michael J. Buckley describes the rise of toleration, and of atheism itself, as a response to religious violence in the preceding years: the expulsion of the Huguenots from France, the Spanish inquisition, the witch trials, the civil wars of England, Scotland and the Netherlands. Buckley argues that "religious warfare had irrevocably discredited confessional primacy in the growing secularized sensitivity of much of European culture." This is a view echoed by Ole Peter Brell and Ray Porter. Marisa Linton, however, points out that it was a common conception that religious diversity would lead to unrest and possibly civil war.Linton, 158.

According to Justin Champion, the question in England was not one of determining religious truth, whether or not there was a god, but rather one of understanding how the priesthood had gained the power to determine what was accepted as truth.Justin Champion, The Pillars of Priestcraft Shaken (Cambridge: Cambridge University Press, 1992), 7. Republican radicals like Henry Stubbe, Charles Blount and John Toland understood religion as a social and cultural institution, rather than as transcendent principles.Champion (1992), 134. They were primarily motivated by priestly fraud or "priestcraft". The second half of Thomas Hobbes' book Leviathan contains an example of this sort of anticlerical thought. Hobbes, like Toland and other anticlerical writers of the period, understood religion in terms of history. By viewing religious truth and the church as separate, they helped open the way for further religious dissent.

Because France was an absolutist monarchy in which the king was seen as ruling by divine right, it was generally thought that French people had to share his religious views. The Edict of Nantes, which granted toleration to the Huguenot minority in France, was revoked in 1685. Marisa Linton argues that while the philosophes did contribute to some extent to the rise of French toleration, the activities of French Huguenots also played a part: they began to worship more publicly in the more remote regions of France, and their continued loyalty to the French crown on the eve of and during the Seven Years' War may have helped to ease the monarch's suspicions about their faith.Linton, 169 In the mid-eighteenth century, Jansenist intellectuals began campaigning for religious toleration for Calvinists.Linton, 170. Linton argues that together, these causes shifted public opinion towards religious toleration.Linton, 172. Religious toleration was not accepted by everyone; for instance, Abbé Houtteville condemned the rise of toleration in France because it weakened ecclesiastical authority and encouraged irreligion. However, in 1787 Louis XVI granted an Edict of Toleration acknowledging their civil rights to marry and own property, although they were still denied the official right to worship and could not hold public office or become teachers. Full religious toleration for Protestants would not be granted until the French Revolution.

Writers on toleration The Dutch Jew Spinoza argued for individual freedom to express personal beliefs, while discouraging large congregations unless they belonged to a somewhat deistic idealized state religion. According to Spinoza, freedom of thought, speech and expression were the core values of toleration—as such, Spinoza opposed censorship. Jonathan Israel summarized his position, that anti-toleration laws were engineered "for personal advantage but also at great cost to the state and the public", and that they exacerbated religious conflict rather than diminishing it. Spinoza constructed his theories about toleration based on a freedom to think rather than the right to worship, and was established according to philosophical principles rather than being based on any interpretation of scripture.Justin Champion, "Toleration and Citizenship in Enlightenment England: John Toland and the Naturalization of the Jews, 1714–1753" in Toleration in Enlightenment Europe, ed. Ole Peter Grell and Roy Porter (Cambridge: Cambridge University Press, 2000), 135. Consequently, Spinoza was essentially arguing for everyone, atheists, Catholics and Jews included.

Pierre Bayle was a strong advocate of tolerance, the basis of a quarrel with Louis XIV.Marin Fitzpatrick, "Toleration and the Enlightenment Movement" in Toleration in Enlightenment Europe, ed. Ole Peter Grell and Ray Porter (Cambridge: Cambridge University Press, 2000), 36. He even defended the idea of an ethical atheist society in his famous dictionary. Martin Fitzpatrick credits him with making a "powerful contribution to the way philosophes would wage war on intolerance and superstition".Fitzpatrick, 36. Although he wanted to diminish the influence of Spinoza, Bayle was treated in a similar fashion by the Huguenots of the United Provinces, who saw him as a dangerous thinker and a potential atheist.

John Locke suggested a pragmatic view of toleration,Tomaselli, 93 although he advanced a concept of toleration only between certain Christian sects. He vehemently denied the atheists' right to toleration since they did not believe in a god, practiced no recognizable form of worship, and were not seeking to save their souls. He similarly denied toleration to Catholics on the grounds that papal authority made them a danger to the state. In essence, Locke advanced a freedom of worship, not a freedom of thought. The vast majority of eighteenth-century writers, like Locke, had no interest in granting religious tolerance to ideas that deviated from the core of revealed religion. Most of these writers were strongly opposed to Spinoza's ideal of toleration, which is "chiefly about individual freedom and decidedly not the freedom of large ecclesiastical structures to impose themselves on society".

Voltaire, in his 1763 "A Treatise on Toleration", continued in the tradition of John Locke, arguing that toleration allowed communication and good relationships between differing confessions in the marketplace.Grell and Porter, 4–5 Allowing the Huguenots to return to France would boost the French economy.Sylvana Tomaselli, "Intolerance, the Virtue of Princes and Radicals" in Toleration in Enlightenment Europe, ed. Ole Peter Grell and Ray Porter (Cambridge: Cambridge University Press, 2000), 93. He would not be the only one to espouse this viewpoint.

Opponents tended to conflate the views of those who wrote in favour of toleration under the heading of dangerous anti-orthodoxy and atheism, despite their radically differing viewpoints and confessions.

Related philosophical movements

Deism Deism is the philosophical belief in a deity based on reason rather than religious revelation or dogma. It was a popular perception among the philosophes, who adopted deistic attitudes to varying degrees. Deism, in this respect, is very different from atheism, which denies the existence of a deity altogether. Voltaire, for instance, was convinced that the existence of god was a demonstrable fact. The deistic god, however, often bore little resemblance to the God of Christian scripture, which meant that deists were often heavily criticized by the adherents of confessional faiths and could be accused of atheism.

Deists often pushed for religious toleration, a move which would have supported the open expression of atheism. This is not because they supported atheism—they did not—but because deist philosophers tended to be in favour of the civil freedom of conscience. As Michael J. Buckley writes, "If atheism was unacceptable, superstition and fanaticism were even more so." Deists were not pro-atheist, but their anticlerical leanings indirectly benefited the evolution of atheism.

In historiographical terms, it has been quite common to see a close link between deism and atheism. Buckley critiques Peter Gay's view of the direct tie between deism and atheism, writing, "the vectors which Gay charts are certainly there, but the distinction may be somewhat too neat, too overdrawn." Louis Dupré describes deism as "the result of a filtering process that had strained off all historical and dogmatic data from Christian theology and retained only that minimum which, by eighteenth-century standards, reason demands."Louis Dupré, Religion and the Rise of Modern Culture (Notre Dame, Indiana: University of Notre Dame Press, 2008), 43 Atheism is perhaps the same process taken a step further. Buckley credits the rise of atheism with the gradual submission of theology to philosophy—as thinkers, including church leaders, began to argue religion on philosophical terms, they opened the way for disbelief—they made atheism thinkable. Deism is, in this perspective, a complicated waypoint on the path to atheism: deism is the philosophical belief in a deity based on reason. Once belief in God is based on reason, it becomes thinkable to reason one's way into disbelief.

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