What was the chief advantage the nation of Israel had?
The chief advantage Israel possessed was that God committed His Word to them. As Paul writes in Romans 3:2, they were entrusted with the Word of God, which placed them in a preeminent, privileged position above other nations. This was their greatest advantage.
Les explains that Israel's chief advantage was not merely the temple, the Shekinah glory, the miracle-working power of God, or the priesthood, though they had all these. The preeminent thing was that unto them were committed the Word of God (Romans 3:2). This is what Paul alludes to when he asks what advantage the Jew had. It placed Israel in an enviable, preeminent position, feasting at Jehovah's table and dwelling on the root and fatness of Abraham, because they were so privileged.