The word unclean is used more than one hundred times in Leviticus 11–15. It is an apt description of the condition of the people; they were morally unclean because of their failure to obey God's commands. The law of Moses was issued, first and foremost, to reveal the holiness of God. The Ten Commandments, as well as the ceremonial and civil laws, were designed to keep God's people distinct from the surrounding idolatrous nations. These laws made a clear distinction between what was clean and unclean. But Israel could not keep these laws perfectly. As a result, the people were spiritually unclean, each and every one of them:
For I am the LORD your God. Consecrate yourselves therefore, and be holy, for I am holy. You shall not defile yourselves with any swarming thing that crawls on the ground. For I am the LORD who brought you up out of the land of Egypt to be your God. You shall therefore be holy, for I am holy." This is the law about beast and bird and every living creature that moves through the waters and every creature that swarms on the ground, to make a distinction between the unclean and the clean and between the living creature that may be eaten and the living creature that may not be eaten. —Leviticus 11:44–47
In this representative text, God called His people to be holy, separated from all that is unclean (19:2; 20:7, 26). Through dietary laws and religious rituals, God was teaching them the necessity of being set apart from the defilements of the world. MacArthur comments, "Sacrifices, rituals, diet, and even clothing and cooking are all carefully ordered by God to teach them that they are to live differently from everyone else. This is to be an external illustration for the separation from sin in their hearts." But no one could keep these laws and regulations perfectly; to break one point of the law was to be guilty of it all (James 2:10). The law was a continual reminder to the Israelites of their uncleanness as they stood before their holy God. Every part of the divine law was an indictment of their sinfulness. Thus, the law testified to their moral separation from God.
This excerpt is taken from Foundations of Grace by Steven Lawson.