The short answer
Voltaire did not write this. It was composed in 1906 by Evelyn Beatrice Hall, writing under the pen name S. G. Tallentyre, in her book The Friends of Voltaire. She intended it as her own summary of his attitude toward a banned book — a description of a position, not a transcription of his words.
How a summary became a slogan
Hall later explained that she had been paraphrasing, not quoting. By then it was too late: the sentence was too good, too balanced, and too useful. It has been attributed to Voltaire on placards, in parliaments, and in countless online arguments ever since — and continues to be misattributed today, sometimes by very prominent people, occasionally alongside lines that came from far uglier sources.
Why this one refuses to die
Because it performs a difficult idea perfectly. Free-speech arguments are hard to compress: you must convey both disagreement and protection in a single breath. Hall's sentence does it with a two-part structure — concession, then commitment — that English speakers find almost irresistibly satisfying. A well-built sentence outlives the question of who built it.
Giving credit where it is owed
There is a quiet injustice here worth correcting. A woman writing under a male pen name produced one of the most influential sentences in modern political language, and history handed the credit to the famous man she was writing about. Naming Hall costs you nothing and is simply accurate.
What this means for your English
Study the shape: “I disapprove of X, but I will defend Y.” That concessive structure — acknowledge, then pivot — is one of the most powerful moves in English argument. Learn to build your own sentences on that frame and you will sound measured rather than combative, which is often the difference between being heard and being dismissed.