- Men are good through virtue and virtue is given to man through God.
- Men profit only by following the law. They follow the law because they are good. Being good happens before someone follows the law.
- The law deals with common good. Some people follow that common good but in personal matters are bad. The law doesn't make them good.
- Some laws are bad and tyrannical. These obviously don't make man good.
Aquinas quotes Aristotle as saying "the intention of every lawgiver is to make good citizens". He goes on to say in his own words:
For if the intention of the lawgiver is fixed on true good, which is the common good regulated according to Divine justice, it follows that the effect of the law is to make men good simply.
My own sense is that concepts like law, justice and good are very well intertwined with each other. The law gives guidelines as to correct behavior. For example, speed limit laws aren't universal. They aren't any kind of law that occurred before cars were developed. Before then, people would drive their horses and carts at whatever speed seemed appropriate and things were fine.
The law was introduced and people had to adapt to it. If they broke the speed limit law, they weren't necessarily immoral, but by following it as a guideline, they developed the common good that is a safe traffic place. That means that they may sacrifice some speed (a kind of profit) for a greater good.
Again though, let me note that this isn't a universal law. Speed limits vary and in some places they don't have them at all. The people there aren't 'bad' but they do belong to a different driving culture. (I can't help but think of my brother who was stationed in Germany for a while. He would be horrified that I'm holding up speed limit laws as an example of law providing a good guideline!)
I think laws help people be good but on their own, they aren't reliable enough to create good behavior. Laws against murder are fairly universal but if some clerical error struck murder laws from the books, I don't think many people would go on killing sprees. Cultural elements and teaching of virtue have some (possibly greater) role than the actual text of the law does.