Augustine’s fundamental insight, against Pelagians or Arminians (full, semi-, demi-semi-, or whatever) is veridical: theology that makes my conversion, or my subsequent persevering in sanctity or growing in it, dependent on my own decision to seek holiness or on my own sanctified decisions or actions, must “beware, lest … the grace of God be thought to be given somehow in accord with our merit, so that grace is no longer understood as grace” [De praed. sanct. 1.6]. There is indeed no escaping the logic: if at any step or stage of spiritual life my choice or action determines whether or not I am in fact to be sanctified, then indeed that is what it does, and God’s role can only be to confirm my choice. Which is to say, God’s grace is not free, and so is neither God nor grace.
Augustine did not cultivate this logic for its own sake, but as a pastor, for the comfort of the bewildered North African believers of his time. They compared themselves to martyrs and other spiritual heroes of the just previous age of persecution, and had to doubt the worth of their own choices and actions; that is, if Pelagius was right, they had to doubt the possibility of their salvation.