"He Who created thee without thee, will not justify thee without thee." [Sermon 169; quoted here]
The point, of course, is that God does no violence to the human will when he saves us. We must consent, and it must be a real consent. Amusingly, I've seen some Protestants say silly things like this: "This notion of free will is clearly quite different from that of Augustine and St. Thomas Aquinas." I say it's amusing and silly, since a) Augustine said it, and b) Aquinas appealed to this quotation as an authority in defense of his views on justification and baptism (see above link to the Summa)!
In point of fact the Catholic Church affirms the fact that our wills are free (§1711) and the fact that God has an eternal plan of predestination (§600), and her faithful sons do the same notwithstanding the difficulty (impossibility, in my case!) of comprehending this mystery.
Of course, we've seen in various posts in my series on Aquinas' view of justification that God's grace precedes any movement of our wills towards himself, so that we cannot pretend (a la the Pelagians) that by our natural powers alone we can attain justification: we can't. Nevertheless, no one is going to go to hell in spite of himself; no one is going to go to heaven against his will, either. God respects the fact that he has made us with a nature possessing a free will, and he does not violate it even when he moves us by grace to love him and hate sin.