Here is a quote from B.B. Warfield, from his essay “Augustine”, which can be found in the book, Calvin and Augustine. It is a beautiful declaration of God’s grace. To Warfield, Augustine’s big contribution to the church is the recovery of the doctrine of God’s grace.
It would be altogether a mistake to suppose that Augustine consciously discriminated between the theology of grace, which was his personal contribution to Christian thought, and the traditional Catholicism, which he gave his life to defend and propagate. In his own consciousness, the two were one: in his theology of grace he was in his own apprehension only giving voice to the Catholic faith in its purity. Nevertheless, however unconsciously, he worked with it a revolution both in Christian teaching and in Christian life, second in its depth and its far-reaching results to no revolution which has been wrought in Christian feeling and thought in the whole course of its history. A new Christian piety dates from him, in which, in place of the alternations of hope and fear which vex the lives of those who, in whatever degree, hang their hopes on their own merits, a mood of assured trust in the mercy of a gracious God is substituted as the spring of Christian life. And a new theology corresponding to this new type of piety dates from him; a theology which, recalling man from all dependence on his own powers or merits, casts him decisively on the grace of God alone for his salvation. Of course, this doctrine was not new in the sense that it was Augustine’s invention; it was the doctrine of Paul, for example, before it was the doctrine of Augustine, and was only recovered for the Church by Augustine.