We all make decisions every day that impact us in a negative or positive way. These can be financial in the form of how we spend money, social in the form of how we choose to interact with people, or political in the choices we make to be informed about strategic issues and write our representatives, cast our votes, and let our voices be heard. These are just a few of many examples, but all of these decisions have logical consequences that are beneficial or not so beneficial; if I choose to spend my pay check on DVD's, my kids won't have enough to eat, but if I carefully make a budget and stick to it, my family has all we need to get along.
Aside from the obvious and practical consequences, we are also subject to the tyranny of our own conscience. If an atheist chooses to break God's law by murdering another or stealing his goods, even if he is not caught by the authorities and punished, his own conscience will condemn him! That is not to say that over time, one practicing theft will not grow callous and have an easier time committing the crime.
Conversely, the individual and society as a whole benefits from living according to Biblical principles and listening to their conscience, whether they are trying to glorify God or not.
The Bible explains the role of the conscience this way:
(Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law, since they show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts now accusing, now even defending them.)
I can't help but think of O.J. Simpson who for all intents and purposes did get away with murder by escaping legal consequence. But his life since then reflects a man who is certainly not content and I am inclined to think he carries a very guilty conscience with him wherever he goes and maybe even wishes he had been convicted and been sent to prison to pay for his crimes. As anyone who has violated their conscience knows, it can be terrible.
The fact is our actions have consequences and these consequences apply universally, and we are aware of right and wrong from a very early age. My toddlers remind me daily of this as they demonstrate an awareness of what is sin. In their childish folly, they choose to do what they know is wrong because they simply enjoy it! It is then my job to introduce a consequence that is proportional to the pleasure of that act so they choose not to go that route next time around. This is done in hope that they will choose to avoid much more serious consequences later in life.
Whether you acknowledge a Creator or not or have any belief in a universal literal moral code such as the Bible, this awareness of sin proves that morality exists. As to where the laws come into the picture, this can be treated as a "chicken and egg" problem, as some believe the laws of our land establish a moral code. I don't believe this moral code was created as much as our laws reflect and reinforce existing social taboos that originate in the conscience. That conscience that is created by a very real and all powerful God.
I believe morality is created by God and designed to preserve our precious liberty and this is absolutely universal. So essentially, you are living by the "rules" whether you believe in them or not.