From Acts 15:

[James said] “my judgment is that we should not trouble those of the Gentiles who turn to God, but should write to them to abstain from the things polluted by idols, and from sexual immorality, and from what has been strangled, and from blood. For from ancient generations Moses has had in every city those who proclaim him, for he is read every Sabbath in the synagogues.”

Then it seemed good to the apostles and the elders, with the whole church, to choose men from among them and send them to Antioch with Paul and Barnabas. They sent Judas called Barsabbas, and Silas, leading men among the brothers, with the following letter: “The brothers, both the apostles and the elders, to the brothers who are of the Gentiles in Antioch and Syria and Cilicia, greetings. Since we have heard that some persons have gone out from us and troubled you with words, unsettling your minds, although we gave them no instructions, it has seemed good to us, having come to one accord, to choose men and send them to you with our beloved Barnabas and Paul, men who have risked their lives for the sake of our Lord Jesus Christ. We have therefore sent Judas and Silas, who themselves will tell you the same things by word of mouth. For it has seemed good to the Holy Spirit and to us to lay on you no greater burden than these requirements: that you abstain from what has been sacrificed to idols, and from blood, and from what has been strangled, and from sexual immorality. If you keep yourselves from these, you will do well. Farewell.”

Where do these requirements come from? They come from a section of Leviticus 17 and 18 (17.10-18.18) which spells out prohibitions on blood and on incest. All too commonly pagans in the Greco-Roman world both ingested blood for alleged benefits and also married siblings. Both of these things were prohibited in the Law of Moses and Gentiles were included. 17.10 begins:

If any one of the house of Israel or of the strangers who sojourn among them eats any blood, I will set my face against that person who eats blood and will cut him off from among his people.

The law against consuming blood goes back to Noah. The law forbidding inceset, however, was given through Moses for the first time as far as we know. (Get that? We owe the basic morality of the developed world not to Noah or general revelation but to Moses!)

But right in the middle of that section, between the prohibitions on blood and the prohibitions on incest we find this passage:

And the LORD spoke to Moses, saying, “Speak to the people of Israel and say to them, I am the LORD your God. You shall not do as they do in the land of Egypt, where you lived, and you shall not do as they do in the land of Canaan, to which I am bringing you. You shall not walk in their statutes. You shall follow my rules and keep my statutes and walk in them. I am the LORD your God. You shall therefore keep my statutes and my rules; if a person does them, he shall live by them: I am the LORD.”

So Leviticus 18.5 is involved in responding to the question of justification! Consider the other two uses. First from Romans:

For Christ is the end of the law for righteousness to everyone who believes. For Moses writes about the righteousness that is based on the law, that the person who does the commandments shall live by them. But the righteousness based on faith says, “Do not say in your heart, ‘Who will ascend into heaven?’” (that is, to bring Christ down) or “‘Who will descend into the abyss?’” (that is, to bring Christ up from the dead). But what does it say? “The word is near you, in your mouth and in your heart” (that is, the word of faith that we proclaim); because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved.