John Chrysostom
After telling the Gentile that he will be cut off if he does not continue in faith, and telling the Jews that they too will be grafted in if they do not continue in unbelief, the Apostle passes over the more unpleasant matter in silence. He mentions only the more pleasant one, and so ends his discourse, giving the Jews great hopes, if only they themselves should will it.
Therefore he continues: "For I do not want you to be ignorant of this mystery, brethren, lest you be wise in your own opinion." Here he calls a mystery what is unknown and hidden, and what contains much that is wondrous and incomprehensible. So he says elsewhere: "Behold, I tell you a mystery: we shall not all sleep, but we shall all be changed" (1 Cor 15:51).
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What, then, is this mystery? "That blindness in part has happened to Israel." Here again he strikes the Jew, though he seems also to humble the Gentile. By this he means the same thing he said above: unbelief did not come from all the Jews, but only from a part.
So he also said: "If anyone has caused grief, he has not grieved me, but in part, that I may not burden you all" (2 Cor 2:5). Elsewhere he says: "When I have first enjoyed your company for a while" (Rom 15:24).
Here too he expresses the same thing he said above: "God has not cast away His people whom He foreknew," and again: "Have they stumbled that they should fall? Certainly not" (Rom 11:2, 11).
Here he confirms precisely this: not the whole people has been rejected, but many have already believed or are to believe afterward. Since he has announced something great, he brings forward as witness the prophet who says the same.