Do you realize who came up with that theory? (Jesuits were spreading it before Darby)
Christ is the true Israel and anyone looking to him for salvation, forward in time in the OT or back to the cross now, are the true nation of Israel. To say it is not so is close to a denial of Christ.
In Hosea 11:1, Hosea predicted a time when “Israel was a child, I loved him, and out of Egypt I called my son.” But in Matthew 2:15, the evangelist tells us that Hosea’s prophecy was fulfilled when his parents took Jesus to Egypt to protect him from Herod’s “slaughter of the innocents” (Matthew 2:3-18). Yet, after Herod had died, God called Jesus and his family to return to Nazareth. Matthew takes a passage from Hosea, which clearly refers to Israel, and tells his reader that this passage is now fulfilled in Jesus Christ! He does this to prove to his largely Jewish audience that Jesus is the servant of the Lord, foretold throughout the Old Testament (especially Isaiah).
This is the point that Paul is making in Galatians 3:7-8, when he says “know then that it is those of faith who are the sons of Abraham. And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, `In you shall all the nations be blessed.'”
Paul then goes on to say in Galatians 3:29, that “if you are Christ's, then you are Abraham's offspring, heirs according to promise.”