Jansenism is probably more appropriately called Cyranism after the Abbe Saint-Cyran. While the group did collect around the work Jansen wrote, Saint-Cyran was more influential in its workings as he was the one who began Antoine Arnauld on his path defending St. Augustine/Jansen.

The heresy/controversy of Jansenism was in two parts: first, that on the issues of grace and predetermination, and secondly within the political realm.

The ideas of grace are too complex to do justice here, but in short, Jansen supported the ideas of Michael de Baye, or Baius who believed that men were innately evil, that Jesus only saved a few, and that the only grace of any worth that man receives from God is that which man does not deserve. Baius was declared a heretic, so part of why Jansen would be declared a heretic would be due to his defense of Baius. Part of Jansen's argument was in direct contrast to Molinism, the theological teachings of the Jesuits, which called for a "middle science" to explain God's grace and how He predetermines things.

Complicating the matter was the fact that both Molina and Jansen were out of line as the Pope had called for an universal silence on the matter when Baius had first released his work, the Concordia. All parties involved got in trouble for breaking this silence, but the Jansenists especially were targeted since the Jesuits and Dominicans were both favored by the Church.

As for the political side, even this controversy was divided between Jesuits v. Jansenists, and Jansenists v. the King and the Papacy and everybody else. The Jesuit/Jansenist conflict was wrapped up mainly in the grace issue. However, as the Jesuits gained political influence and prestige, they would be able to use that power to further their fight against the heretics.

As mentioned earlier, the movement would possibly have been better named Cyranism since it would not gain steam until Cardinal Richelieu imprisoned Saint-Cyran for various reasons, none dealing with his beliefs or his fight against the Jesuits, but all for his politics. The final straw was when he would not give his blessing on a divorce for a noble. However, by throwing Saint-Cyran in jail, Richelieu created a martyr.

To keep from copying over my entire senior thesis, I shall be brief. For the two centuries following the start of the Jansenist movement, the group would find most of its support in times of chaos among the nobility. One such time was after the death of Louis XIII, for without his death the Fronde would not have occurred, during which several of the Jansenists' leading protectors, especially the Mme de Loungueville, would gain power, but also the wrath of Louis XIV. This wrath would also apply to the Jansenists, which would lead to the razing of Port-Royal--their headquarters--with Loungueville's death. Louis XIV would bring them to a near death, with his death allowing a resurgence that would continue until the Jesuits were kicked out of France and the Church and the coming of the French Revolution.