So really I think St Augustine - aside from the obvious spiritual heights he achieved - was an amazing guy. Like wildly smart. When you read some of his work on music and how his conception of music and numbers preceded later mathematicians (and philosophers) concepts of irrational numbers by centuries, it's pretty incredible.
However, I think he struggled with trying to express a systematic theology for things he had experienced but lacked the nuanced language of the Greek fathers to describe. I get super cautious here, because St Photios said that much of what St Augustine wrote was stretched later to other people's ends, but it seems that he perhaps in chasing the error of Pelagius used language in error, and his way of expressing his arguments led theologians in the West to overstate and misuse his writings. However, he didn't logic his way there, but was struggling to express his understanding of the heresy of Pelagius as best as he could. It should be noted that at the end, St Augustine was right and Pelagius was wrong.
But St Augustine wasn't simply disagreeing with Pelagius on the grounds of philosophy or a systematic and synthesized theology -- he was speaking from experience of God. If you read what he says, the real root of his objection to Pelagius is that he was not expressing a true experience of God as St Augustine had experienced.
I think this should always be the way we read and compare any theologians work. The commonality of expression of theology "from within God" as St Gregory says is the hallmark of trustworthy work.
