The key figure in the French Enlightenment known for his advocacy of civil liberties is Voltaire. A prolific writer and philosopher, Voltaire criticized established religion and promoted freedom of speech, tolerance, and reason. His satirical works, including "Candide," exemplify his belief in liberty and individual rights. Voltaire’s ideas laid the groundwork for modern democratic principles and human rights, influencing the French Revolution and inspiring subsequent social and political reforms across Europe.