Commentaries:
"It is not that the word of God has taken no effect" must be understood in light of why he is writing this. The silent question is, "What about Israel?" as God seems to be setting Israel aside. It appears as if God has been a failure in His dealings with Israel. He gave them the Covenant, but the people did not want to keep it. But Paul argues that, no, Israel is not being set aside. He is saying, "No, you don't really understand what's going on." It is not that the word of God has taken no effect.
"For they are not all Israel who are of Israel." Who is a Jew? Who is really part of Israel? Only those whose heart has been circumcised (Romans 2:29)! If a Gentile is circumcised in the heart, in God's eyes he is an Israelite!
John W. Ritenbaugh
The Covenants, Grace, and Law (Part Eleven)
Other Forerunner Commentary entries containing Romans 9:6:
Genesis 35:10-11
Matthew 10:5-7
Luke 16:22
John 3:7
Galatians 4:24-26