Jansenism was a 17th -- 18th century Catholic religious and political movement centered in Bourbon France. Cornelius Jansen, Bishop of Ypres in the early 17th century, attempted to reconcile Roman Catholic and Calvinist theology. Jansen preserved the Mass and sacraments, but reinterpreted them according to aspects of Reformed Christian theology. Jansen contradicted orthodox Catholic theology by preaching a number of doctrines at odds with the Vatican's dogmatic definitions at the Council of Trent.
Orthodox Catholicism teaches that all persons are capable of receiving sacramental grace, but only after confessing their sins. Jansen taught that certain people were elect and certain damned according to God's creation (predestination). Those in mortal sin could be filled with "irresistible grace" even without confessing their sin.
Orthodox Catholicism teaches that God will save all who repent and partake of the sacraments worthily (in other words, Jesus died for all people so that all persons may be saved). Jansen taught that Jesus died only for the elect (as John Calvin likewise taught).
Jansen parted with Calvin over any foreknowledge of election. Jansen was convinced that there was no way that a person could know of his or her election.
Jansenism was incredibly popular in 16th century France. Blaise Pascal was an ardent follower. Jansenism also took hold in seminaries, monasteries, and convents. From there, Jansenism spread to England and Ireland. Exiled English and Irish Roman Catholic men often trained for the priesthood in northern France.
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Eventually Jansenism was suppressed by Pope Clement XI in his bull Unigenitus (1713). Nevertheless, the Jansenist ethos of simple worship (emphasis on said Masses), pietism (emphasis on the rosary), and asceticism, survived particularly in Ireland. This is why American Catholicism does not have the emotional exuberance of Mediterranean Catholicism.
With a POD of the mid 17th century, integrate Jansenist teachings within mainstream Catholicism. In particular, the Sacred Heart devotion will need to be butterflied away.