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Have you ever considered justification through Jesus to be one of the ways God shows his oneness AND that he is the one true God? In Romans 3:21-26, Paul makes three points about what God shows us regarding his character: God manifests his righteousness through Jesus' faithfulness (the literal phrase in these verses is "the faith/faithfulness of Jesus") and the redemption that comes through him, rather than the Law (verses 21-24); God publicly displayed Jesus as the true "hilasterion," which is the Greek word for the mercy seat in the Old Testament sanctuary service - the place where you come to meet with a merciful God (verse 25); and God demonstrates his righteousness through Jesus so that he can be the just God and the one who justifies all of us through Jesus (verse 26).
Then, Paul makes an interesting move: having stressed that nobody can boast because we are justified by faith instead of our performance or obedience (Romans 3:27-28), he concludes that God is not only the God of the Jews, but also of the Gentiles (verse 29). Why does Paul draw this conclusion?
It's because of what he says in verse 30: "Since indeed God who will justify the circumcised [i.e., Jews] by faith and the uncircumcised [i.e., Gentiles] through faith is one" (NASB). According to Paul, God shows he is the one true God by justifying both Jews and Gentiles from the same faith - the faith that comes from Jesus! Likewise, God shows that "God is one," like Deuteronomy 6:4 tells us, by showing he is the same God to Jews and Gentiles - he doesn't treat them differently, but gives them the same faith in the same Jesus!
So what does this mean for you? It means that God is reliable and you don't have to worry that he might treat this person differently from how he treats you. He loves you, and them, and everyone else, and he shows that same love to each of us. And it also means that God is interested in bringing people together so they can be one with each other, and in this way he can share his oneness with us.
Friends, let us trust in the God who justifies Jews and Gentiles alike, for this God always exhibits the same reliable, trustworthy love toward all of us.