Augustine argued for free will, but only as compatible with God's foreknowledge.
"God must needs have given free will to man. God's foreknowledge is not opposed to our free choice." (On Free Choice of the Will, Book Two, I, 7, Book Three, IV, 38)
He found God blameless for the evil He knows will happen. He solves the problem of evil by blaming man.
"God foreknows all the things of which He Himself is the Cause, and yet He is not the Cause of all that He foreknows. He is not the evil cause of these acts, though He justly avenges them. You may understand from this, therefore, how justly God punishes sins; for He does not do the things which He knows will happen." (Book Three, IV, 40)
Augustine argued for his undeniable existence with a form like that of Descartes' famous Cogito, Ergo Sum.
In respect of these truths, I am not at all afraid of the arguments of the Academicians, who say, What if you are deceived? For if I am deceived, I am. For he who is not, cannot be deceived; and if I am deceived, by this same token I am. (City of God, Book XI, Ch. 2)
Augustine analyzed Latin words to find the meanings behind them
And how many things of this kind does my memory bear..., they must as it were be collected together from their dispersion: whence the word "cogitation" is derived. For cogo (collect) and cogito (re-collect) have the same relation to each other as ago and agito, facio and factito. But the mind hath appropriated to itself this word (cogitation), so that, not what is "collected" any how, but what is "recollected," i.e., brought together, in the mind, is properly said to be cogitated, or thought upon. (Confessions, Book X, xi)