The very next chapter is chapter 4, which is all about God and Abraham — a midrash (Jewish form of commentary), as it were, on a part of the Torah. Paul is about to explain how he upholds the Torah. Paul isn’t upholding the food laws and Sabbath observance. Rather, he’s upholding the entirety of the Torah — by explaining how the Torah’s presentation of God’s covenant with Abraham is being fulfilled. He’s upholding the Torah because the Torah tells the story of God, a story that is coming to fruition through Jesus’ apostles. He upholds the Torah because the Torah itself speaks of a new age when God will circumcise the hearts of his followers — an age that has finally arrived!
“Then what becomes of our boasting? It is excluded.” How? Well, because this is not a system based on merit, true, but also because this is not a system based on ethnicity. The Jews can no longer boast in being sons of Abraham over against the Gentiles, because the Gentiles have been invited in through faith in Jesus (“grafted” in per Romans 11).
Now, “justified” or “declared righteous” is a judicial, legal term. It means the declaration by a judge of “innocent.” If you are tried and found innocent, you are “justified,” that is, found righteous, and “justification” is the judge’s pronouncement that you are innocent of the charges.
But “righteous” also takes us all the way back to —
(Gen 15:6 ESV) And he believed the LORD, and he counted it to him as righteousness.
Abram was justified, that is, declared righteous by his faith.
But we can’t overlook this passage —
(Gen 18:17-19 ESV) 17 The LORD said, “Shall I hide from Abraham what I am about to do, 18 seeing that Abraham shall surely become a great and mighty nation, and all the nations of the earth shall be blessed in him? 19 For I have chosen him, that he may command his children and his household after him to keep the way of the LORD by doing righteousness and justice, so that the LORD may bring to Abraham what he has promised him.”
God chose Abraham so that his descendants would “do righteousness and justice” so that God could honor his promises to Abraham. God’s calling of Abraham was for a purpose, and that purpose includes creating a people who act like God. After all, “righteousness” and “justice” are two key characteristics of God.
Paul’s argument is thus that because Abraham’s descendants did not do righteousness and justice, Jesus had to die on the cross “so that the LORD may bring to Abraham what he had promised him.” Jesus would be a light of the world because the Jews failed to do so. And Jesus’ disciples would also be a light of the world, because God’s righteousness and Jesus’ faithfulness would create a new people out of both Jews and Gentiles who would be a people of faith, declared righteous. And by the power of God himself, through his Spirit, these people are to do righteousness and justice — to be like God.