Today at church we will read from Leviticus. Leviticus has an extensive amount of material on ancient Jewish sacrificial rules. Most Christians that I know have never read it and never will. That includes any number of people who claim that it is "the word of God."
Leviticus 18 begins with God talking to/through Moses to His people. He tells them, "be holy because I (God) am holy." The people of Israel are told that they can not do like the Egyptians do or do like the Canaanites do. This command from God introduces three chapters on what the "doing" of Israel should look like. As a preacher, it raised for me a couple of issues, especially since it is linked to Jesus' teaching in the Sermon on the Mount.
The Law of Moses and the "Law" of Jesus both assume that God is holy and that we are supposed to be holy. Holy means, among other things, to be set aside for God. We are consecrated to Him and intended to be used as He sees fit. So holiness runs contrary to the popular expression "it is my body and I can do with it what I want." [theologically, not politically]
Because God is the one who sanctifies us, it can be said that holiness is a gift. It is unearned. It is not something that accomplish for myself. Jesus makes that abundantly clear as He explains that murder is also anger/ill will in the heart, that adultery includes lustful desires, etc. So we are, by grace, made part of God's people. We are holy.
But holiness is not magic. Holy objects can be misused. Holy places can be desecrated. Holy people can do blasphemous things. Hence, the list; an explanation of what we should and should not do. The list is controversial in places. Chapter 18 is all about sex. There is a long list of people who are forbidden as sex partners. It focuses on incest, but also includes homosexuality and bestiality. Chapter 19 is more about personal morality and social justice. I am not getting into that debate.
Anyhow, the point of the hoiness code is that God is holy and He wants us to be holy. And the further insight is that being holy entails doing (and not doing) concrete actions. Judaism, and Christianity, have connected holiness with religion. This is simply not the case in other religions of the ancient near east. In fact, the pagan gods are frequently sensualists who are immoral. The neighbors of Israel did things that God found offensive.
So as you look in the mirror today, understand that if you are part of God's people you are holy. And if you are holy, then you are supposed to do holy things. And holy things, if the Bible is true, include sexual morality, not lying, not stealing, treating people fairly, providing access to the poor. Being holy means you "do" forgiveness, and you "don't do" revenge and hate. As Leviticus says, and Jesus repeats, "you love your neighbor as yourself."
Holiness means you worship (at church!). It also means you work hard to be good and you work hard to make life good for those around you (including people you call enemies!). Being holy means "do" what you "be." (Think of it as good fruit from a good tree. Apple trees make apples. Christian trees make Christian acts). In discerning what actions to engage in it is important first to think: "Because I am holy, I will/will not do this." What would the world look like if Christians acted holy?