Pelagius is associated with the idea that we, as humans, are free to self-determine, in the sense that we have the power to choose freely between good and evil.
St Augustine was not so sure. He believed that what lay behind the choices we make was more complex and that a "freed will" required both knowledge and feeling to be integrated. This integration of the human will was made possible "by an inseparable connection between growing self-determination and dependence on a source of life that always escapes self-determination".
I'm not enough of a theologian to be fully aware of the implications of this concept of the role of grace. But it does seem to have led Augustine to a sophisticated concept of freedom in which our efforts to self-determine take place, inseparably, in relationship with something that we know is not ours to will. And so freedom is not to be understood in terms of choice:
Freedom, therefore, for Augustine, cannot be reduced to a sense of choice: it is a freedom to act fully. Such freedom must involve the transcendence of a sense of choice. For a sense of choice is a symptom of the disintegration of the will: the final union of knowledge and feeling would involve a man in the object of his choice in such a way that any other alternative would be inconceivable.
(Source: Peter Brown, Augustine of Hippo: A Biography, p.376)