And if some of the branches were broken off, and you, being a wild olive tree, were grafted in among them, and became a sharer of the root and the fatness of the olive tree with them, do not boast against the branches. But if you boast, it is not you that bears the root, but the root bears you. You will say then, The branches were broken off so that I might be grafted in. Well, because of unbelief they were broken off, and you stand by faith. Do not be high-minded, but fear. For if God did not spare the natural branches, fear lest He also may not spare you either!If God has not preserved national Israel, the natural branches, then there is no such thing as "Preservation of the Saints." You can't have it both ways.
--Rom. 11:17-21
So....God stopped preserving His saints for almost 1900 years??
Both are God's people--national Israel is the root and the natural branches, and we Gentiles are grafted into that root by God's grace. Your theology is equivalent to saying that because I have an adopted brother, I am no longer my parents' firstborn.
No, it's simply recognizing that God wasn't lying when he said "They are not all Israel who are called Israel." Your theology is ignoring the fact that we Christians are the spiritual seed of Abraham who will receive the covenant promises. The inferior covenant has given way to the superior one.
Nowhere does the Bible teach that national Israel is the root of anything. This is a sad twist on reality by (semi-)dispensationalists.
The faith of Abraham, and the Seed, Jesus Christ, are the root of true religion, not national Israel. "Natural branches" (Jews) only share in the blessing and nourishment of the root as they trust in Messiah for their redemption. This is true also for wild branches, the gentiles. There is no distinction in Scripture.