A major belief of Voltaire was Freedom of Speech. This basically meant that you should be allowed to say whatever you wanted without being punished.
Another main belief of Voltaire was Freedom of Religion. This meant that people could believe in or practice any religion that they wanted without being punished or forced to believe in something different.
Last but not least, Voltaire believed in separation of church and state. This meant that the church shouldn't be able to be the government, shouldn't enforce laws, or should force people to attend church
A few of his books are Candide, Zadig, Letters on the English, and the age of Louis XIV.
He was also well known for his quotes such as "I disapprove of what you say, but i will defend to the death your right to say it" and "Common sense is not so common"