(Proverbs 28:4 KJV) They that forsake the law praise the wicked: but such as keep the law contend with them.
We are supposed to be a nation of law and be ruled by law. What does that mean? It means that our society has agreed on certain types of behavior. Some actions are acceptable; others no. Right and wrong are defined and the standards are written down. They are not to be changed to benefit one powerful individual. They are not to be ignored when it is convenient. In a broad sense that is the idea here. As Christians we are to be law abiding.
We see that concept comes from the Bible. The law referred to here is not the civil code that tells you not to park in front of a fire hydrant. It is talking about the law of God which is demonstrated by, but not limited to, the Ten Commandments. The principle is rooted in the Hebrew tradition. God does not change. Right and wrong do not change. There are situations where it might be hard to make a decision but the unchanging principles are always there to guide us.
So? Again, how can you know the rules unless you read the book? How can you know what is wicked and who is wicked? We are to support the righteous and abandon the evil. It is the law.