When he was young Voltaire was educated by the Jesuits at Collège Louis-le-Grand, who taught him Latin, theology and rhetoric.
During his lifetime Voltaire published numerous books, plays, essays, and poems displaying his prodigious wit and satire in such works as the novel Candide, the short story Micromega and the fictitious Lettres philosophiques.
Although Voltaire was not a philosopher in the strictest sense, he was a prominent figure in the European intellectual movement known as the Enlightenment.
His writing was considered controversial because he criticized political institutions, the authority of church and the power of clerics. In turn, he advocated for freedom of speech, freedom of religion and separation of church and state.
Voltaire ardently believed in the power of reason. He felt that social progress should be advanced through reason, not religious or political authority. Through his writing he promoted the concepts of liberty, equality, and progress.
Voltaire also opposed most religions and religious institutions. He put forth that the Bible and the Quran were outdated scripts and because they were written by mortals; he deemed they were not divine works.
While it may appear that Voltaire was an atheist, he was not. He was a deist who fervently advocated for deism in both England (where he lived in exile for years) and France.
Deism is the belief in the existence of a supreme being who does not intervene in the universe or the affairs of mankind. Voltaire believed that an all-knowing deity could be understood through observation and reasoning, rather than the blind faith which most religions promulgate. In his Treatise on Toleration (1763) he defends the right of religious freedom for all declaring that all people are of the same God, therefore there should be no division or conflict based on religious beliefs.
When it came to morality Voltaire adopted a position somewhere between the rationalist materialism and the transcendent spiritualism of contemporary Christian theologians.
For Voltaire, human beings are not deterministic automatons, but beings that possess free will. Human beings are also subject to natural and moral laws that can be ascertained through reason and rational deduction. At the same time, Voltaire understood many are incapable of such discernment and self-control so religion can serve a purpose in managing social order.
In his largest philosophical work Dictionnaire philosophique (Philosophical Dictionary) published in 1764, Voltaire criticizes French political institutions, the Bible and the Roman Catholic Church.
In Candide (1759), one of his best known satirical works, Voltaire weaponizes his wit against science, religion, government, philosophy, and literature. He also attacks the metaphysical and philosophical optimism of philosopher Gottfried Leibniz. In particular, Voltaire's criticism is directed at Leibniz's principle of sufficient reason which puts forth that nothing can be as it is without a reason for it being so. The consequence of this principle is the belief that all human conditions, good or unfortunate, are part of a grand cosmic plan.
During Voltaire's period of Enlightenment philosophie he espoused what would become his most influential stance. He declared himself a member of the 'party of humanity' and committed to waging war against fanaticism and superstition. This stance entails the coupling of philosophy with social criticism and reformist political action and is captured by his famous motto "Écrasez l'infâme!" (Crush the infamy). This would become his most lasting and significant contribution to philosophy.
In no small measure, Voltaire's philosophical social position influenced Karl Marx's famous thesis that philosophy should aim not merely to interpret the world, but aspire to change it. In other words, the philosopher should evaluate the value of any philosophy by its ability to affect social change.
Ultimately, Voltaire's philosophical legacy lay in how he directed his philosophical activity rather than in any particular doctrine. It was the explicit intellectual positions he took and how he used his influence to promote them that encapsulated what was known as Voltaire's Enlightenment philosophy.