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Romans 11:18 do not boast against the branches. But if you do boast, remember that you do not support the root, but the root supports you.
The Jews for the most part would not accept their messiah. The gentiles by baptism into Christ became heirs of the promise made to Abraham the father of the Jews. Paul is warning against arrogance and despising of the Jews who failed because of unbelief. Their fall is only temporary; when Christ returns they will recognise their messiah and be reconciled to God. Gentiles should not consider themselves superior to the Jews but rather be careful lest they themselves fall into the same error of unbelief. Salvation comes through the Jews; Christ is a Jew. The Jews, the natural branches are cut off and the Gentiles are grafted into what is Jewish stock and they should not forget it.
John 4:22 "You worship what you do not know; we know what we worship, for salvation is of the Jews.
History shows that the Christian church took little notice of Paul’s words. Their treatment of the Jews has for the most part been exactly what we are being warned against. Paul goes on to expand what he means:
Romans 11:19 You will say then, "Branches were broken off that I might be grafted in." 20 Well said. Because of unbelief they were broken off, and you stand by faith. Do not be haughty, but fear. 21 For if God did not spare the natural branches, He may not spare you either. 22 Therefore consider the goodness and severity of God: on those who fell, severity; but toward you, goodness, if you continue in His goodness. Otherwise you also will be cut off. 23 And they also, if they do not continue in unbelief, will be grafted in, for God is able to graft them in again.
The Jews were blinded by unbelief and the gentiles had their eyes opened. It should not induce a feeling of superiority but rather of humility. The promised salvation will not come until the time of the restoration of the kingdom to Israel arrives.
Romans 11:26 And so all Israel will be saved, as it is written: "The Deliverer will come out of Zion, And He will turn away ungodliness from Jacob;
The Deliverer of the gentiles is also the deliverer of the Jews.
I hope this helps.