Saint Augustine was a unique bishop. He popularized an ethical system that, obviously, revolved around God. It is fairly simple, so I won't bore you with any drawn out introduction on St. Augustine or anything like that.

First off, God created us. This makes us good along with everything else since we were created from pure good, which is God. Evil is only our distorted view of good. We can only find happiness in God since he created us. you see, all of this seeking for happiness and love from things on this earth, such as things, people, ourselves, is incomplete. We can only receive so much love from these things. God fills that gap. Our need for infinite love can only be filled by infinite love, which only God can give us. If we love God, we will only do good since in order to love God we must do what he wants. This means you must follow the Ten Commandments (notice the order. Also note that this is the Jewish version, I think, so that is why it is written the way it is):

  1. I am your lord, do not worship false gods
  2. Do not take God's name in vain
  3. Keep holy the seventh day
  4. Honor thy father and mother
  5. Do no murder
  6. Do not commit adultery
  7. Do not steal
  8. Do not bear false witness against your neighbor
  9. Do not covet thy neighbor's spouse
  10. Do not covet they neighbor's belongings

Notice how your duty to God are the primary commandments, then to your parents. The commandments were written in that order for a reason. If you waiver from your following of God you lose your connection to God, meaning that you could go off and do what ever you want, and that is not a good thing. So according to St. Augustine, as long as you follow the Ten Commandments and find love in God, you can only do good.