Why can't you mix God's dealings with Israel and God's dealings with the Church?
God's dealings with Israel and the Church cannot be mixed because they are two totally different groups: Israel is God's earthly people, while the Church is the heavenly Body of Christ. Their identities and destinies differ, so blending these separate dealings confuses God's distinct purposes.
Les teaches that there is a fixed division between God's dealings with the earthly people of Israel and His dealings with the heavenly people, the Body of Christ, the Church. He says they are so totally different that they cannot be mixed. Israel is earthy; the Church is heavenly. He connects this to the believer's present condition: as we have borne the image of the earthy through Adam, we are earthy now, but we will not stay earthy. Our prospect is to bear the image of the heavenly and one day be transformed to have bodies like Christ's. That heavenly destiny belongs to the Church, not to Israel's earthly program, so the two must be kept distinct. He illustrates this by pointing to Christ's forty days after resurrection as a glimpse of how we will function in eternity.