Here's the thing...I think the first four chapters of Paul are summed up tidily in the seventh verse of chapter one:
"To all God's beloved in Rome, are are called to be saints: Grace to you and peace from God our Father and the Lord Jesus Christ."
Paul writes to the Romans, and spends the first four chapters of this epistle working on the idea that God is particularly invested in the salvation of all in the Eternal City and the earthly precincts over which it holds temporal sway. Jew or Gentile, God's attention (and thus judgment) is upon all. It might differ in nature, for Israel is under the Law while the other nations are not; but make no mistake,
God loves all nations and desires them to hear and receive the Gospel, the Good News of Jesus the Christ. Moreover, Paul is just the sort of apostle who is uniquely imbued with the traits needed to walk with ALL Romans as they seek God's righteousness. He is a Jewish man who happens to also be a Roman citizen. He is a provincial who also has been formed and educated in a very cosmopolitan fashion. He can talk and talk and he attempts to get his audience to walk the walk of the way of Jesus.
And so, we get to hear about Abraham, and the covenant God struck with our common progenitor. Before there was the Law, there was a relationship between humanity and the Almighty. The pride of being set apart that many of Paul's former contemporaries resided in was only an outward sign of Abraham's willing submission to God. By seeking God's will first, and by accepting God's Promise as fulfilled even "when he was as good as dead (for he was about a hundred years old), or when he considered the barrenness of Sarah's womb" (iv, vs 19b), he sets a posture of connection to God that becomes the taproot of Paul's coming apologia for the Gospel of Jesus.
Thus, God is not just the God of Judaism. God is not just one more god amongst all the other gods of Rome and the Empire. God is the God of all human beings, and thus the one who justifies people's faith by grace.
It's more than affirming, "I'm in," when it comes down to it... It is recognizing that we have been in from the beginning, and now is the time we are going to be waking up to a more primary and more challenging connection to God as Father, Son and Holy Spirit...The One who was and is, and is to come. The One who lifts us all up into salvation