Francois-Marie Arouet (French: [fRa.swa ma.Ri aR.we]; 21 November 1694 - 30 May 1778), known by his nom de plume Voltaire (; French: [vol.te:R]), was a French Enlightenment writer, historian and philosopher famous for his wit, his attacks on the established Catholic Church and Christianity as a whole and his advocacy of freedom of religion, freedom of speech and separation of church and state.
Adopts the name Voltaire
The author adopted the name Voltaire in 1718, following his incarceration at the Bastille. Its origin is unclear. It is an anagram of AROVET LI, the Latinized spelling of his surname, Arouet, and the initial letters of le jeune ("the young"). According to a family tradition among the descendants of his sister, he was known as le petit volontaire ("determined little thing") as a child, and he resurrected a variant of the name in his adult life. The name also reverses the syllables of Airvault, his family's home town in the Poitou region.  Richard Holmes supports the anagrammatic derivation of the name, but adds that a writer such as Voltaire would have intended it to also convey connotations of speed and daring. These come from associations with words such as voltige (acrobatics on a trapeze or horse), volte-face (a spinning about to face one's enemies), and volatile (originally, any winged creature). "Arouet" was not a noble name fit for his growing reputation, especially given that name's resonance with a rouer ("to be beaten up") and roue (a debauche).  In a letter to Jean-Baptiste Rousseau in March 1719, Voltaire concludes by asking that, if Rousseau wishes to send him a return letter, he do so by addressing it to Monsieur de Voltaire. A postscript explains: "J'ai ete si malheureux sous le nom d'Arouet que j'en ai pris un autre surtout pour n'etre plus confondu avec le poete Roi", (I was so unhappy under the name of Arouet that I have taken another, primarily so as to cease to be confused with the poet Roi.) This probably refers to Adenes le Roi, and the 'oi' diphthong was then pronounced like modern 'ouai', so the similarity to 'Arouet' is clear, and thus, it could well have been part of his rationale. Voltaire is known also to have used at least 178 separate pen names during his lifetime.

Q: When did Voltaire change his name?
A: The author adopted the name Voltaire in 1718,

Q: Why did he decide to change his name?
A: following his incarceration at the Bastille. Its origin is unclear.

Q: Why was he incarcerated at the Bastille?
A: unknown
How did he choose the name Voltaire?
Its origin is unclear.
Its origin is unclear.