Cornelius Jansen was a Dutch theologian and bishop of Ypres. He is best known for his role in the Jansenist movement, which emphasized the doctrine of predestination and the idea of grace. Jansen's most famous work is "Augustinus," a book that sought to revive the teachings of Saint Augustine. His ideas sparked controversy within the Catholic Church and led to the condemnation of Jansenism as a heresy. Jansen's influence on theology and religious thought continues to be studied and debated to this day.