Why couldn't the earthly Kingdom come to fruition until the Gentile complement was completed?
According to Ephesians 1:10, the promised earthly Kingdom could not be realized until the Gentile complement—the Church—was also completed. Only then, in the Kingdom economy, would everything come under Christ's headship, joining Israel's earthly people and Kingdom with the heavenly Body of Christ.
Les explains that the Old Testament promised this glorious Kingdom would be on the earth—Heaven on earth—because Christ is the King. The King was in the midst of Israel, and Israel was being offered the opportunity to enter the earthly promises that would come to fruition. But Les points out that even according to Ephesians 1:10, this could not happen until the Gentile complement had also been completed, which was the Church. When the Kingdom economy comes, everything will come under Christ's headship: the earthly people of Israel with their earthly Kingdom, and all the ramifications of the heavenly people, the Body of Christ. He then sets the Body of Christ aside for later and moves to Matthew 19, where Peter asks what they will have for forsaking all.
That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him: