Why did God set Israel aside and graft in the Gentiles?
God set Israel aside because He could not bless Israel and the rest of the world at the same time. Since Abraham was promised a blessing to all nations, God broke Israel off from Abraham's root and, by grace, put the Gentile system into the place Israel had enjoyed.
Les explains that Israel was in the promises given to Abraham, but God had also told Abraham He would be a blessing to all the rest of the world. Because God cannot do it two times at once, He set Israel aside, broke them off from the fatness of the root of Abraham, and put in the Gentile system as a group. The whole Gentile system has now, by the grace of God, been put into the place Israel enjoyed in the Old Testament. Les points out that this did not mean all Israelites were believers; most in the Old Testament were renegades in unbelief, yet they were on the root of Abraham. Gentiles today are in the same situation, resting on the root of Abraham. That does not make them Jews; they merely feast off what God promised Abraham. As the Gentile system feasts on the blessings of the Father of Israel, the Gospel goes to the Gentile, who has just as much opportunity at salvation and a relationship with God as Israel did.