The work of the council of Trent consisted in maintaining with the same force two principles, believed to be equally necessary and equally true.
On the one hand, grace is omnipotent, and its spontaneous call is needed to enable man to enter on the way of salvation. On the other hand, man is free and his free consent is needed to enable grace to accomplish its work within him.
What would logic have to say to such a doctrine as this, which apparently combined contradictory elements? Many accounted that logic, in spite of its resistance, was bound to give way, on the ground that two truths of equal certainty could not really be incompatible. Others sought to do away with one or other of the two terms.
Thus it was that Balus, reverting to the strictest doctrines of Saint Augustineof Hippo, asserted the radical impotence of fallen humanity.
Original sin consists in concupiscence, of which baptism takes away the guilt, but not the malignity. The impulses of this concupiscence, even though involuntary, are sins.
The Jesuit Luis de Molina, on the contrary, following the lead of Blessed John Duns Scotus, endeavoured to screen free will from the tyranny of grace. According to him, effectual grace does not differ essentially from prevenient or sufficient grace, being effectual not of and by itself, but only by the adding there to of the free consent of the human will.
It lies with ourselves whether grace shall become effectual or remain simply sufficient. Thus does free will co-operate expressly with grace. God proposes, man disposes, as taught the stoics of old.
In this doctrine, which spread very quickly, Jansenius saw a deadly blow to Catholicism. It was in his view a covert revival of the early Pelagianism; and Pelagianism, through the medium of Origen, had come down in a direct line from pagan philosophy. Seneca had said: "We owe it to the immortal gods to live; to philosophy to live rightly."
This same pride of man, uplifting himself against God, yea, evem above him, was in the eyes of Jansenius the very kernel of the Molinist theology.
On the other hand, Jansenius could neither concede to the protestants that God himself makes man to sin, nor to Balus that he can be said to have sinned when the will to do so was not present. He resolved then to avoid both pitfalls by a strict following of Saint Augustine.
Link (here) to the book entitled, Pascal