I suggest it is mistaken that Paul left them hanging until chapter 9. The advantage given them is implied with 2 additional and significant points Paul makes in C-3. First, God's Law revealed the knowledge of sin (3.20); second, through the Law and Prophets, God's righteousness is revealed in Jesus (3.21-22; cp. Gal. 3:23-25).
I suggest it is mistaken that Paul left them hanging until chapter 9. The advantage given them is implied with 2 additional and significant points Paul makes in C-3. First, God's Law revealed the knowledge of sin (3.20); second, through the Law and Prophets, God's righteousness is revealed in Jesus (3.21-22; cp. Gal. 3:23-25).