I propose that vs.6 and beyond is implicitly responding to three sets of questions (the first two suggesting 'failure' in God's Word whilst the third resembling an charge dealt against Him):
Merely being physically born as a Jew doesn't count for a lot - it is the promised children who are Abe's offspring. What does all this mean? How does one actually become a 'promised child'?
Well, given that we should spread our qualitative scope of exegesis as wide as possible, let's check out what else the book of Romans says about the 'promised children' and being 'Abraham's offspring'.
Rom 4:3, "'Abraham believed God and it was credited to him as righteousness."Rom 4:5, "...to the man who does not work but TRUSTS God who justifies the wicked, his faith is credited as righteousness."
Rom 4:11, "(Abraham) is the faither of ALL who BELIEVE but have not been circumcised...and also the father of the circumcised who WALK IN THE FOOTSTEPS of the faith..."
Rom 4:16, "...the promise comes BY FAITH, so that it may be by GRACE and be guaranteed to all ABRAHAM'S OFFSPRING"
Rom 4:18, "Against all hope, Abraham in hope BELIEVED and SO BECAME the father of many nations..."
Rom 4:20-24, "Yet he did not WAVER THROUGH UNBELIEF regarding the promise of God, but was strengthened in his faith...being fully persuaded that God had power to do what He had promised. THIS is why 'it was credited to him as righteousness...these words were written also for us who BELIEVE IN HIM who raised Jesus our Lord from the dead."
Given the above it should be clear that one of Paul's goals is to
remind everyone that God cannot be said to have failed because it was Israel
i.e. Man(!), who screwed up through UNBELIEF. It just won't do to be born
as a Jew, outwardly circumcised and be part of the 'Chosen Race' - if you
still treat God with indifference and distrust, things are still gonna
go sour. It is only through FAITH that one becomes a true offspring of
Abe.
As John the Baptist scolded the Pharisees in Matt 3:9, "Produce fruit in keeping with REPENTANCE...for out of these stones God can raise up children of Abraham!", one must clearly distinguish the SUPERNATURAL/SPIRITUAL dimension from the NATURAL / PHYSICAL one. Being of the latter earns the Israelites no cigar - they must trust in God in order to be counted among the former.
Thus in vs.7-9 the apostle speaks necessarily of the vast difference between being GOD'S children (which comes through repentance, trust and faith), and merely having similar DNA. In other words, yes, God did elect them as a physical nation to represent Him but, failing to trust Him, many of them bungled miserably to be ultimately reckoned as Abraham's offspring.
On to verses
10-13...