Book of Leviticus #12
Before spelling out the laws of conduct for families and, by extension, the lay society, the Lord begins by defining what the family is and is not.
Chapter 18 of the Book of Leviticus has the most complete treatment of incest and forbidden sexual unions in the Old Testament. This may strike us as odd; after all, why not speak of other sinful behavior such as those related to food, or money, or abuse of power, why start with sexuality?
The simple, and straightforward answer is, that sexuality is integral to the definition of the family, and, the family, when seen from the Cross, is the greatest natural sacrament for the Holy Trinity and the Kingdom of God.
Indeed, the Lord has written three bibles, not just one: the cosmos, the family, and the sacred scriptures. The family is an image of the Trinity and must reflect that image so that society at large can be prepared to receive the Good News and nurture it. If the family is broken or worse disfigured, then the image of God is disfigured, preventing his children from recognizing him.
This is why the Lord begins, in this chapter, by establishing the boundaries of the family, and he does this within the confines of the Old Testament when polygamy was tolerated ("for their hardens of hearts" the Lord will explain later).
By studying this chapter, we gain critical insight into the way the Lord views the family and the importance it plays in the salvation of mankind, an insight that leads us into the heart of the Church who is the one and only Bride of Christ