Augustine, while granting God the first and last say in making the rules, argues that by a simple measure of doing good by not doing wrong to others or giving in to the excesses that bring man earthly pleasures without glorifying God, man can praise God and live happily on earth.
He does give much understanding to the types of evils that man commits, and allows that sometimes what might be considered in strictest form against the laws of God, is not a sin based on the intent. This does show a better understanding of how the nature of man is often in conflict with what God intended as right, and in a manner of speaking, Augustine is recognizing man’s free will.
He reiterates that the basic laws of God are as applied to all men, regardless of their own manmade script. This is something that I personally have been wrestling with, the idea that if a religious belief is founded within a sect, the members are bound by that and non-members are not and yet Augustine confirms the ethical belief that this is not the basis of ethics.
In saying that vicious acts contrary to human customs are to be avoided, we take account of variations in custom, so that the mutually agreed convention of a city or nation, confirmed by custom or law, is not to be violated by the lust of a citizen or a foreigner. (III.15)
Augustine raises an odd question here that I don’t quite get:
But when you (God) suddenly issue a command which departs from customary expectation, even though at one time you forbade the doing of any such act, though for a time you conceal the reason for your authoritative verdict, and though it may go against the agreed customs of a given human society, who would hesitate to say that your command is to be kept? (III.17)
I’m not sure how we come to understand or know when God’s changed His mind and amended the laws. Any understanding would come through man’s interpretation, perhaps instigated by changes in his world that would normally necessitate these changes, but here too, it would be of man, not of God. For example, I would assume that God’s supposed directive "Go forth, be fruitful and multiply" might be due for an overhaul.