Augustine's discussion of free will in Book V of the City of God is fascinating. I'm not going to be able to unpick it all in this context and I think it would be immodest to try but some features immediatly strike me. The most important of these is the importance Augustine gives to the word 'free' and to the word 'will'. We need to place this in a context though. Augustine's purpose in his argument about free will is to take on two enemies. He defines them as Cicero and the Stoics- we may see the shades of Christian dispute here too. The first error, associated with Cicero, is to say that there is a God and belief in him is a good. If something is good as opposed to bad, it must have been chosen. If something is chosen the choice is unpredictable until it is made. (V 9) Therefore there can be no foreknowledge of the world. The second error is the Stoic error. That is to say that there is a fate which determines everything- so there is foreknowledge but there is no freedom. (V 10)
Augustine is attempting to show in these brief passages that both perceptions are wrong. Many scholars have laboured over precisely what he means here. I want though to pick out two aspects of his argument. The first is his concentration and obsession with the concept of will. Augustine is incredibly interested in the will and he locates all agency within the universe within wills. So he states 'material causes... can do only what the wills of spirits do by means of them'. Material things are made but do not make, spirits on the other hand 'both make and are made'. (V 9). The distinction is important. Augustine is basically drawing a distinction here between something that is conscious and intends to do something to the world and something that is unconscious and is shaped but will not shape the world. This is an attack on any idea of man's passivity or resignation before the world: Marcus Aurelius, Augustine would argue, believes in trying to make himself into some kind of thing whereas man and angels, like God, are able to shape the world they live in.
Secondly Augustine discusses the power of our will to remake the world. Here he attempts a definition of man's will as against God's. So God's will is defined by its supremacy and ability to finally shape the world, man's by its subservience, willed or otherwise, to God's. God knows the definition of each man's will before he begins his life and by that definition, a definition that is eternal, knows what man will do. As Augustine says 'our wills also have just so much power as God willed and foreknew that they should ahve. Therefore what power they have they have most certainly: for He Whose Foreknowledge cannot fail foreknew that they would have the power to do it and would do it' (V 9). Here the will is being defined but notice its freedom is not abstracted: it can still will but it will not will in a particular way because it has no power to do so. Augustine makes this simpler in his next section: where he asks the question about whether God is free to die. The question is an illusion, God cannot die because that is one of his properties: he is free to die in the sense that he might if he so wished but it is not in his definition to so do.
This is important. This argument from definition allows Augustine to ellide foreknowledge and freedom of choice. He puts this at its clearest here:
For a man does not sin because God foreknew that He would sin. On the contrary, ther is no doubt that the man himself sins when he sins. For He Whose foreknowledge cannot fail foresaw not that fate or fortune or something else would sin, but the man himself. If a man chooses not to sin, he certainly does not sin; but if he chooses tos in, this also was foreknown by God. (V 10)The idea is that it is in the properties of the man that he would chose to sin. He chooses to sin and really makes a choice but the way that he was defined, the way that he was placed within the world, meant that in that situation he would sin.
As I said this is an imperfect rendering of a complicated subject: but I think it is in this area of will, definition and freedom that Augustine is able to argue both for divine foreknowledge and for human choice.