In defending the Church against Pelagianism, Augustine took the extreme stance of denying any role at all of man's free will in his salvation. He once wrote, for example:
Will you dare say that even when Christ prayed that Peter's faith
might fail, it would still have failed if Peter had willed it to fail?
As if Peter could in any measure will otherwise than Christ had wished
for him that he might fail.
On Rebuke and Grace, Chapter 17
Although Augustine appeared not to be fanatical about his views, others in the Church - especially Prosper of Aquitaine - adopted his extreme anti-Pelagian views. Other Church Fathers such as John Cassian argued that grace and free will must be viewed in the context of a sort of synergy between God and man, but Prosper and others dismissed this view altogether. Prosper wrote, for example,
By a sort of contradiction, there is taught that ... [some] from the
endowments of free will, have this desire to seek, to ask and to knock
Contra Collatorum, Chs 2:2, 2:4
Augustine himself modified his views in later works. In The Call of All Nations he writes that he sought "to investigate what restraint and moderation we ought to maintain in our views in this conflict of opinions" (Book I.1). He emphasized that grace does not compel man but acts in harmony with man's free will:
If we give up completely all wrangling that springs up in the heat of
immoderate disputes, it will be clear that we must hold for certain
three points in this question. First, that God wills all men to be
saved and to come to knowledge of truth. Secondly, there can be no
doubt that all who actually come to the truth and to salvation, do so
not in virtue of their own merits but of the efficacious help of
divine grace. Thirdly, we must admit that human understanding is
unable to fathom the depths of God's judgments.
This was a considerable modification of his previous view that grace was not only a help, but the sole agent in man's salvation. The Canons of the Council of Orange reflect Augustine's modified views. While firmly emphasizing the role of God's grace, they do not explicitly deny that the actions of man himself also have a role in his salvation. The Canons simply state that man cannot be saved solely through his own will, which was essentially the Pelagian thesis. What probably troubled Warfield was that the Council of Orange ascribed any importance at all to man's free will.
[Most of the above follows the explanation of the Pelagian controversy found in Seraphim Rose's book, The Place of Blessed Augustine in the Orthodox Church]