Holiness to be maintained because Jehovah is holy
Chapters 19-20 carry us somewhat further. They were to be holy, for Jehovah was holy. Chapter 19 takes up rather the side of good, though keeping themselves from all that was profane, or profaning what was holy; but we find what is good and kind and comely, what ought to be their conduct, in various details, in the relationship they sustain one with the other, either with regard to various dangers to which they were exposed in their walk, in their everyday circumstances: for they had to do with God, and Jehovah was their God. The people of God were, in all their ways, to walk in a manner worthy of this relationship, and even to understand what was suitable to man, to every relationship in which they were found, according to God. Thus, though it was not here priesthood, it was the practical maintenance of this relationship with Him who dwelt among them, and to whom they drew nigh, by guarding against defilements unsuited to those who were in it. It is here we find the precept to love our neighbor as ourselves.
Separation from idols and evil because Jehovah had sanctified the people
Chapter 20 guards more against the evil and corruption which was found among the nations. In both they are called to be holy, in chapter 19 more in conformity to the character of God, in chapter 20 to keep apart from idols and evil because Jehovah had sanctified them to Himself. It insists upon purity in every respect.