In fact, the demonic powers will go to any lengths to increase our rebellion against the one true God, even to the point of faking divinity and creating a great sham battle so that the Romans will think the pagan gods are at war and be inspired to write bad poetry (or even good poetry) about it. That the Romans then copy the 'gods' in their own civil wars can hardly be surprising.
This in contrast to the law of Christ, which restrains vice and promotes virtue...
Again, Augustine rejects the idea that there are secret moral teachings passed on to the select few. While all of us retain a sense of right and wrong (however much we may voluntarily pursue wrong in the end), none of us can claim to have picked that up from the mysteries of the 'gods.' All we get from them is public lewdness and encouragement to pursue our depravity on a mass scale.