And you shall not walk in the customs of the nation that I am driving out before you, for they did all these things, and therefore I detested them.
As Israel moved into the land promised to them by God (the promised land) they were to carefully avoid taking on the customs, traditions, and rules of the nations that they were conquering. The main reason was that God hated their evil ways and customs which dishonoured him through their worship of false Gods. God did not want his earthly representatives to start acting different to how he had commanded and taught them.
The lesson for Christian's today is to be careful how we shape our lives as we continue our lives in the world which is fallen and corrupted by the earthly desires of the majority who do not follow Jesus. Rather than adopting customs and traditions through inattention, accident, and even deliberate choices made from the part of us that is still being reformed; we should be careful to make our lives deliberately into something which we know is pleasing to God.