When we take pity upon a man and care for him, it is for his advantage we do so; but somehow or other our own advantage follows by a sort of natural consequence, for God does not leave the mercy we show to him who needs it to go without reward. Now this is our highest reward, that we should fully enjoy Him, and that all who enjoy Him should enjoy one another in Him. [On Christian Doctrine, I.32; emphasis added]
So we see once again that St. Augustine was not some sort of proto-neo-crypto-quasi-Protestant. Protestants (generally) deny that the good works of Christians merit reward, an error that Trent rightly condemns. But St. Augustine affirms that our works will receive a reward. He was a Catholic.