(kôrnā'lĭs yän'sӘn), 1585–1638, Dutch Roman Catholic theologian. He studied at the Univ. of Louvain and became imbued with the idea of reforming Christian life along the lines of a return to St. Augustine. He established a close friendship with Duvergier de Hauranne, a fellow student, with whom he shared and developed many of his theological ideas. In 1630, Jansen became professor at Louvain, and in 1636 bishop of Ypres. Out of his lifework, the posthumous Augustinus (1642, in Latin), arose the great movement called Jansenism.
Jansenism was strictly a Roman Catholic movement, and it had no repercussions in the Protestant world. Its fundamental purpose was a return of people to greater personal holiness, hence the characteristically mystical turn of Jansenist writings. St. Augustine's teaching on grace was especially appealing to Jansen, who stressed the doctrine that the soul must be converted to God by the action of divine grace, without which conversion could not begin. Predestination was accepted in an extreme form and was so essential to Jansenism that its adherents were even referred to as Calvinists by their opponents. But Jansenism had no appeal to Protestants, for it held the necessity of the Roman Catholic Church for salvation and opposed justification by faith alone.
Jansenism, however, came into conflict with the church for its predestinarianism, for its discouragement of frequent communion for the faithful, and for its attack on the Jesuits and the new casuistry, which the Jansenists thought was demoralizing the confessional. Jansenism took root in France, especially among the clergy. There it early became involved with Gallicanism, and high officials of church and state often sided with Jansenists to thwart the Holy See.
The second great Jansenist work was De la fréquente communion (1643) of Antoine Arnauld, which stirred the opposition of Jesuits and Dominicans. In 1653, Pope Innocent X condemned five of Jansen's doctrines, and in 1656 Arnauld was expelled from the Sorbonne. Meanwhile, Blaise Pascal, the greatest Jansenist, aroused a storm by his anti-Jesuit Provincial Letters, and there was persecution of the Jansenists for a while. Pasquier Quesnel published late in the 17th cent. a vernacular New Testament with Jansenist notes, which was condemned by Pope Clement XI. The aged Louis XIV undertook to suppress Jansenism, and the bulls Vineam Domini (1705) and Unigenitus (1713) virtually put the Jansenists out of the church. (Gallicanism, however, prevented the legal registration of Unigenitus in France until 1730.)
The convent of Port-Royal, the greatest center of Jansenism, was closed, and most Jansenists fled France. Jansenism survived as a tendency within the church, especially in France, taking the form usually of extreme scruples with regard to communion. In the Netherlands an organization not in submission to the pope was set up. There are Jansenist bishops of Utrecht, Haarlem, and Deventer. The independent Jansenists recognize the Council of Trent and are, except for their special differences, like Roman Catholics. The first Old Catholic bishop was consecrated by Jansenists (see Old Catholics).