John Chrysostom
Then, having reproachfully said, "Do not boast against the branches," the Apostle seems to console the Jew. In reality he shows his low worth and great dishonor.
Therefore he did not say, do not boast, but, "do not boast against the branches." Do not exalt yourself before them as broken-off branches, because you have been placed in their stead and enjoy what belonged to them.
Do you see how he seems to reproach the Gentiles, but in reality wounds the Jews?
"But if you boast," he continues, "you do not bear the root, but the root bears you." What benefit is this to the broken-off branches? None.
As I have already observed, the Apostle seems to devise some weak shadow of consolation. Yet even by that with which he reproaches the Gentiles, he inflicts a mortal blow on the Jews.
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By saying, "Do not boast against the branches," and, "if you boast, you do not bear the root," he showed the Jew that what had happened was worthy of boasting, though one must not boast of it.
By this he awakens and encourages him to faith. He presents himself as his defender, shows him the loss he has suffered, and shows that others possess what was his own.