Much of Paul's writing is informed by his need to address an important concern on the Christian church of his time: Can a non-Jew become a Christian? Here, in this letter to the Romans, he reminds them that, after all, Abraham himself was not Jewish at the time that God chose him to be our great ancestor. Paul makes an explicit distinction between Abraham's faith and someone's following religious instruction.
The modern Christian church, as has the church throughout history, continues to wrestle with the question of who is eligible to be included in our faith community?
For a contemporary example, read United Methodist Church moving toward all-inclusive membership policy.
Who is to be included? "We are," Paul says to his fellows Jews, "because we are descendants of Abraham and God promised inclusion to all of his descendants."
Then Paul adds, "But, remember this: God chose Abraham before the world had even heard of Moses. Abraham couldn't follow the law of Moses before Moses brought it down from the mountain. God's choice was not made because Abraham followed the Jewish law, and it still isn't."
God's promise rests on grace.
Lectio Divina: Psalm 22:23-24