"But the ultimate end of every creature is to attain the divine likeness.... Therefore, it would be incompatible with providence for that whereby a thing attains the divine likeness to be taken away from it. Now, the voluntary agent attains the divine likeness because it acts freely, for we showed...that there is free choice in God. Therefore, freedom of will is not taken away by divine providence.
Again, providence tends to multiply goods among the things that are governed. So, that whereby many goods are removed from things does not pertain to providence. But, if freedom of will were taken away, many goods would be removed. Taken away, indeed, would be the praise of human virtue which is nothing, if man does not act freely. Taken away, also, would be justice which rewards and punishes, if man could not freely do good or evil. Even the careful consideration of circumstances in processes of deliberation would cease, for it is useless to dwell upon things that are done of necessity. Therefore, it would be against the very character of providence if liberty of will were removed."
In other words, God is free to act. Also it is the object or end of our existence to gain the qualities and attributes found in God, thus we must be free to act and inasmuch as we are free to act that cannot be taken away from us by God. The reason why God would not take away our freedom to act is because if He did so then He would be taking away from us something that is good, which would be counter to His nature. Thus we must be free to act (have free will, be able to choose, etc.).
Our freedom to act makes our acts just or unjust, good or evil, virtuous or not. So we are free to be agents unto ourselves, meaning we are free to choose what we shall do, but just because we are free to act does not mean that all of our actions are correct, or good, but rather it allows us to make correct and good choices. This of course assumes that there are right and wrong, good and evil, correct and incorrect, but it is obvious that there must be because we are free to act. If there were no choices given to us then our freedom to act would be null and void, as we would not be able to act. Thus acknowledging the liberty of our will, shows that there must be good and evil, right and wrong, as well as the converse being true (if there is right and wrong, then we must have free will).
What is left is to discover what is right and what is wrong, what is good and what is evil.