Whom, then, does the Apostle mean here, and why did he say this? By the firstfruits and root he means the patriarchs of Israel: Abraham, Isaac, and Jacob. By the mixture and branches he means the whole people of Israel.
He affirms that, if the patriarchs are holy, as indeed they are holy and in glory with God in heaven, then the whole mass of the people is holy.
The Apostle said this to suggest that the whole mass of the people ought to be holy. It was founded as such in the patriarchs, and was appointed and capable of being such.
Therefore, when you now see unbelieving Jews, do not think that they will remain so. The time will come when they too will believe and become holy.
He offers this assurance to confirm that his previously expressed expectation is not vain. They will be received, and after this the glorious Kingdom of God will be revealed following the resurrection of the dead.
Therefore his labors are not vain, by which he seeks to convert the Gentiles more quickly. Through this he assists the Jews' more rapid conversion and the swifter revelation of the Kingdom of God.
Yet as the Apostle extends his thought into the distant future, he certainly does not lose sight of the Jews then present. By this parabolic saying he may also have wished to provoke them to faith.
It is as though he were saying: behold what you are appointed for. Why wait? Turn quickly and enter the rank of the saints, so that you do not remain long in estrangement from your holy root.
Blessed Photius, in Oecumenius, says: "If the mixture of bread and the branches must necessarily be holy with the holy firstfruits and root, and the Jews are not holy because of unbelief, then it is clear that they have fallen away from their ancestral root, Abraham."
To this Oecumenius himself adds: "By calling them holy, he disposes them toward faith, as though saying: you are fit and suited for faith. Only one thing is lacking in you, the desire to believe."