A 250-year-old book by the Enlightenment anti-establishment writer Voltaire is climbing best-seller lists in France weeks after the attacks by French-born Islamic extremists that left 20 people dead, including the gunmen.

The "Treatise on Tolerance" is a cry against religious fanaticism and stemmed from Voltaire's conviction that religious differences were at the heart of world strife. He wrote at a time of bloody tension between French Protestants and Catholics.

The Jan. 7-9 attacks started when two gunmen stormed Charlie Hebdo, a satirical weekly that had received death threats for caricatures of the Prophet Muhammad, decimating the paper's staff. Also attacked was a kosher supermarket, where four hostages died.

Voltaire's book came out in 1763 and now has a place among the bestsellers for Amazon, FNAC and French bookseller Gibert Joseph.