Augustine is a difficult writer, and City of God is one of his most difficult works. The problem, it seems to me, is not that of following his arguments on a line-by-line level, or not primarily. Rather, the problem is figuring out why he is even talking about this topic in the first place. (I suppose we could just dismiss him as a sloppy thinker, etc., but those explanations are never very interesting or compelling to me.)
Nearly every book of City of God seems to be taken up with extraneous material and never get around to its main point — yet clearly Augustine believes that he’s getting at his main point. If you want to get at what’s distinctive in Augustine’s thought, you need to be able to get a sense for the unstated superstructure that is directing his inquiry — you need to develop a scent for the often unstated questions to which his arguments are an answer.
I’m tempted to draw a parallel with Dogopolski’s understanding of the task of Talmudic analysis, which is not to arrive at an answer but first of all to reawaken the disagreement that motivates the text.