What St. Augustine taught - what the Catholic Church teaches is not a difficult concept to grasp. Man cannot do good works without the grace of God, so in that sense we do not have the free will to do
good without God’s grace. But you know what?
“For the grace of God that brings salvation has appeared to all men.” (Titus 2:11)
God offers Christ’s salvific grace to all people, because “God does not show favoritism.” (Romans 2:11) But it is up to us, of our own free will, to cooperate with His grace, that He has freely given us, not based on merit, so that we can stay in His grace.
I’m not sure what you meant above, but I hope you’re not saying that St. Augustine taught “double predestination”, or that he denied free will. He said, in the aforementioned book (Ch.5):
“But who of us will say that by the sin of the first man free will perished from the human race?”
He was responding to Julian (a Pelagian), who also twisted Augustine’s words to make him deny free will, as the Calvinists do, except they do so to support their doctrine. So Augustine’s message here is for them as well (Ch. 9):
“Away, then, with the notion that in my book that man should read anything opposed to these divine testimonies! But either by not understanding, or rather by calumniating, he seeks to twist what he reads into another meaning.”
Now, ironically, I had not read this treatise by Augustine before, and yet…
“But against the Pelagians also, not without reason, we pronounce an anathema, for that they are so hostile to God’s grace, which comes through Jesus Christ our Lord, as to say that it is given not freely, but according to our merits, and thus grace is no more grace; [Romans 11:6] and place so much in free will by which man is plunged into the abyss, as to say that by making good use of it man deserves grace—although no man can make good use of it except by grace, which is not repaid according to debt, but is given freely by God’s mercy. And they so contend that infants are already saved, that they dare deny that they are to be saved by the Saviour. And holding and disseminating these execrable dogmas, they still over and above constantly demand a hearing, when, as condemned, they ought to repent.” (Ch. 42)