Synergism is the belief that Unrealigned humans are fully predisposed to evil except that they can trust God to realign them.
"The beginning [of justification] is to be derived from the prevenient grace of God, [but man] is also able to reject it."
Council of Trent, Session 6, 1547.
Monergism is the belief that trusting God is an effect of realignment, not a cause of it.
"Faith is not the work of men, but of the Holy Spirit."
Martin Luther et al, Augsburg Confession, Item 18, 1530.
Synergism implies that God's utmost is not enough to realign a self, that some contribution, at least acquiescence, from the self is necessary before realignment can occur.
Disguising its Bonhominalism, Synergism purports to admit that man cannot merit God's favour, but proceeds to aver that he can at least choose whether to accept it, thus claiming for man a residual goodness that can, when push comes to shove, make the right choice. Thus man clutches at the straw of his own goodness.
Luther's Monergism was shared by Calvin, the other principal Protestant pioneer. Ironically Luther's students, calling themselves "Lutheran", soon reverted to (a more subtle form of) Synergism, whereas Calvin's, calling themselves simply "Reformed", made less of the pioneers' names but upheld their belief.
Synergism also reappeared among the Reformed in the form of Arminianism. In the early 1600s the Dutchman Arminius, under the influence of the Humanist ideal of "free personality", sought to undermine the meaning of sola gratia by introducing human free-will into the Calvinist creed. He claimed that God gives sufficient grace to everyone, and that whether he grants justifying and then preserving grace depends on the autonomous free will of the individual. The sola gratia formula was kept, but the meaning was radically changed.
At some point Monergism became known as "irresistible grace", which is misleading as it implies that resistance cannot prevail, the real point being that, at the level on which God operates when he realigns a self, resistance does not arise; "efficacious grace" is better, as in the Canons of Dort.
"We believe as a result of the efficacy of God's mighty strength."
Synod of Dort, Canons (1619). (Thus they render Ephesians 1:19; a bad translation, I gather, but I cite it only as exemplary Reformed vocabulary, not as exemplary Pauline scholarship.)
Back to Bonhominalism.