I am reading Jacob Milgrom’s Leviticus, a one volume commentary in the Continental Commentary series.
What sparked my interest in this was Milgrom’s agreement with the theory of a few other Israeli scholars that Leviticus 17 and the chapters that follow constitute a unique stratum of the Pentateuch. They call it the Holiness source. So the old J, E, P, D documentary theory needs to include H as well. Others have posited H, but these Israeli scholars specifically say that H represents a social and moral turn in priestly theology. This revolution in priestly thought took place in response to the concerns of prophets like Isaiah and Micah who cared more about ethics than ritual. It took place in Judah in the years following the downfall of Samaria, the northern kingdom.
From my reading of Milgrom, it looks like the biggest difference between the two parts of Leviticus is that in the first 16 chapters the concern is with the ways that the sanctuary can become unclean. There is a connection between the sanctuary and the nation, but the main concern is dealing with sin’s pollution of the sanctuary. The whole system in these chapters is about how to keep the sanctuary fit of the ritual’s performed there. The sanctuary is a holy place, but the profane world tends to impinge upon it, so the priests must take measures.
In H it is the land that needs to be made holy. It is hard to see a big change if you just read through Leviticus in a linear way. Chapter 17 just reiterates the priestly notion that life is in the blood, which is the basis for the whole system of animal sacrifice. Chapter 18 starts out with a series of commands that establish boundaries for sexual behavior and specifically forbid child sacrifice to Molech. But then it says:
Do not defile yourselves with any of these things, for the nations which I am about to drive out before you have been defiled with all these things. Therefore the land has become unclean and I have brought the punishment for its iniquity upon it, so that the land has vomited out its inhabitants. You yourselves must obey my statutes and my regulations and must not do any of these abominations, both the native citizen and the resident foreigner in your midst, for the people who were in the land before you have done all these abominations, and the land has become unclean. So do not make the land vomit you out because you defile it just as it has vomited out the nations that were before you (Leviticus 18:24-28 NET Bible).
The passage takes the perspective of Moses speaking to Israel before entering the land. But see the theology here that says the pagan way of life had made the land itself unclean. The land, therefore, had “vomited out” the Goyim. Don’t make the land vomit you out too. This is what had happened to not only the Canaanites but also to the Israelites in the north.
In the first chapters of Leviticus it is mainly the priests who have to worry about avoiding uncleanness. They are the ones who mostly could contaminate the Temple. But now all the people–even foreigners who live in the land–have to worry about it. They do not just have to worry about contact with corpses or bodily secretions when they are on the way to the Temple. Now they have to distinguish their life-style (chapter 19 will go on to command social reforms) from that of pagan culture. Otherwise, they will pollute the land itself.