Did Augustine equate free-will with Pelagianism? I thought that the issue between them was on the degree of human depravity? A low opinion of human nature is certainly central to Calvinism. Pelagius had a very different view. He got a warmer reception in the east, because at first sight, it seems close to the eastern notion of synergy. The language that Augustine developed was certainly off-putting to them. But even in his blackest mood, even as his Roman world seems to be disappearing under his feet, Augustine seems less pessimistic than Calvin about the state of human nature. Funny that Pelagius should appear on the scene just as barbarism seems close to overwhelming Roman civilization and thereby darkening Augustine’s mood.
I wouldn't say "low opinion" or "pessimistic view". Rather I would call it a realistic assessment. All one has to do is be honest with themselves in relationship to how much they like to read of God's word, how much they like to pray, or even how much they like to tithe (to name but a few things). If one takes an honest appraisal of themselves, the depths of our sinfulness becomes readily apparent.
But it's a funny thing. The more we understand about and admit to our sinfulness, the more we understand the depths of the love of God.