The question of whether Augustine is a monergist or a synergist is more complicated. For one thing, even at his most monergistic, Augustine does not deny that we are active in our own salvation. Augustine is a monergist with respect to the origin of faith, for instance, in that he sees it as resulting from prevenient or “operating grace” rather than “co-operating grace” (his terms). But for Augustine this does not take away the role of human free will, for what prevenient grace does is precisely to move our wills so that they freely will the good. Hence for Augustine grace never undermines or replaces free will. In that sense he is never a radical monergist, as if the human will had no active role to play. On the other hand, he is indeed a monergist in a less radical sense, because for him the gift of faith is wholly the work of God, since even our freely willing to accept God’s gift is a work of grace alone.
So in that sense, Augustine is clearly a monergist with respect to the gift of faith, unlike the Arminians. Ultimately it is up to God, not us, whether we freely choose to accept what God has to give us. However—and here is the real complication—this does not make Augustine a monergist with respect to salvation. The reason why is that Augustine does not have a Calvinist concept of saving faith. For he does not share Calvin’s distinctive new doctrine about the perseverance of the saints, according to which everyone with true (i.e., saving) faith is sure to persevere to the end and be eternally saved. For Augustine, you can have a perfectly genuine faith but not persevere in faith to the end of your life. There is no guarantee that believers will not lose their faith and thus ultimately be damned. Hence no matter how true your faith presently is, that does not mean you are sure to be saved in the end. Consequently, Augustine’s monergism about faith does not make him a monergist about salvation.
Read the rest here. Ben Myers gives some thoughts on the question here. I am increasingly coming to the conviction that the whole monergism/synergism debate is wrong-headed and that a more refined understanding of creation as divine gift is needed. What we have in relationship with God is a unilaterally established reciprocity. Much of what I encounter in contemporary Calvinistic doctrines of salvation can be regarded as a collapse of reciprocity in a suffocating form of monergism, suggestive of a weak grasp of the significance of our faith in a Triune God.