The Romans had failed to be content with a few gods and apparently needed the protection of as many gods as they could get their hands on. So, they acquired as many gods as they could – gods from lands they conquered and gods from allied lands and gods from trading partners and all kinds of gods. Augustine suggests that they lived better with fewer gods and that the more gods they had, the more immorality they participated in and the more enemies were willing to oppose them.
In chapter 13, Augustine starts talking about how the Romans acquired wives. By force. Not sure what the connection is. He is saying, at least, that getting wives and some of their gods made the Romans more enemies, and that maybe they would have been more just and more happy if they had had fewer gods and found wives in a more just way. Other than that, the chapter seems out of place to me. If anyone has thoughts on Augustine’s purposes here, let me know.