"...for where there is no law there is no transgression." [Rom 4:15]
It is becoming more and more popular for people to say that there are no absolute truths, a theory called "Relativism." The idea is that there are no absolute rights or wrongs and people must do what is best for or brings the most happiness to the most people.
There are only two problems with this theory: 1) It's not true, and 2) The people who claim to believe it really don't. Just wait until someone slaps you or steals from you or kills someone you love, and you don't have to think twice about the fact that they did something wrong. For example, when a politician is caught with his hand in the cookie jar, his supports rush to defend him and find excuses for why what he did wasn't really that bad ("think of all the good he's done!"). But those same people immediately condemn a political opponent who does the same thing. They don't really believe the action was somehow okay the first time, they were just okay with who was doing it.
I think that most would agree, to some degree, that right and wrong is then determined by law. This is true, but whose law? Certainly not the law of man. How can sinful creatures come up with their own, perfect law that defines right and wrong when we ourselves are incapable of following such a law? And besides, different places have different laws. Which are more right or more wrong? And some laws are created for the sole purpose of protecting evil men.
Suppose a group of people from all over the world board a rocket and fly off into deep space, never to return to Earth again. No government is set up and no system of laws are established on this rocket. One day, someone steals a dehydrated food packet from someone else. Is that wrong? Why? These people have no law, and they are no longer under the laws of the lands which they left. Why would such an act still be wrong?
The answer is because it is God's law that ultimately determines right and wrong. Paul said in Romans 4:15 that "where there is no law there is no transgression." If there was no law on this hypothetical rocket ship, then stealing someone's food packet wouldn't be a transgression of the law, and therefore wouldn't be wrong. But there is a law, one that all men everywhere are subject to; God's law.
It is this law that compels us to make our own laws as just as possible. It is God's law that allows us to recognize injustice and evil in the world, even when such things aren't actually a violation of man's law at the time.
Christ said in John 12:47-48 that all men will one day be judged according to his (Christ's) word. Think about how wonderful a thing that is? Could you imagine if we were judged according to the fickle laws of man or what we "felt" was right and wrong? The Nazis couldn't be found guilty, because they were acting in accordance with their own law. A murderer couldn't be found guilty, because he was doing what he thought was right at the time. Having a universal standard actually makes right and wrong easier to understand, because it's the same everywhere at every time.