How did Israel respond when the Messiah came?
For the most part, the nation rejected the Messiah when He came, though a few did believe and say yes in faith. The Law had been intended to prepare Israel to receive Him, but they were not ready, so the nation as a whole rejected the offer.
Les explains that the Law was like a schoolmaster or servant meant to prepare Israel and bring them to the place where, when the Messiah would come, they were ready. In faith they could have said yes, and a few of them did, but for the most part the nation rejected it. They could have believed everything Christ was had they been ready, but they were not. Les then applies the same concept to the Gentile economy, noting the Law was given to prepare for the Messiah and His work of the Cross.