What did Israel do with the blessings and promises they had from Abraham?
Israel enjoyed the promises of Abraham—the fatness of the root—but rejected them in unbelief. They walked them underfoot, cast them aside, and said thanks but no thanks, losing their exalted position of blessing despite having the Word of God and everything going for them.
Les explains that Israel was enjoying the fatness of the roots of Abraham, meaning the promises given to Abraham. But they rejected it in unbelief and lost that exalted position described in Romans 2-3, where they had the Word of God and everything else going for them. Earlier, Les notes they walked it underfoot, cast it aside, and effectively said, 'thanks, but no thanks.' Their unbelief caused them to be broken off as branches, and the Gentiles were then grafted in to partake of the same root and fatness, though Gentiles also failed to take advantage of the opportunity.