God did not look at the fact that they descended from such great fathers and from such a blessed race. He rejected them when, by their unbelief, they became unworthy of mercy.
For there is no partiality with God. Each receives according to what he is worth, either mercy or lack of sparing.
Behold, you, a Gentile, believed and for this were received into the blessed race of God, the race of those being saved.
Reason and keep in mind that if God rejected those whose race He had as His own, and which He had so carefully guarded until now, because of unbelief, then much more will He not spare you.
You are of another race, received from outside into the race of God. If you weaken in faith, or waver in it, or behave unworthily of it, He will not spare you.
Keep this in mind and fear. Fear losing the blessing of which you have been counted worthy, and being deprived of God's mercy.
"Unbelief made the Jews alien to the root. Faith joined you to the root and made you a partaker of its fatness. Therefore one must not be high-minded, but fear and tremble.
Fear what? Fear losing this spirit of faith and therefore being deprived of God's mercy. For if kinship by nature brought them not the least benefit when they did not have one and the same disposition with their fathers, then much more will you become alien to the root if you do not preserve grace" (blessed Theodoret).
"And rightly so. For if God, because of unbelief, cut off those who were worthy by the advantages of their fathers, and to whom the promise of adoption by God had even been given, what will He not do to those who were so exalted without any advantages, if they waver in faith or become proud?
For they were honored not at all according to worthiness" (Ambrosiaster).
The Apostle's meaning in the words, "lest He also not spare you," is that He certainly will not spare you. Yet he expressed himself so irresolutely and hesitantly either in correspondence with the word "fear."
For fear is usually revealed by unresolved expectations: lest such a thing happen, lest one suffer this or that. Or he speaks this way to soften in words the severity of the sentence; see Oecumenius.
Thus he leaves the readers to understand it for themselves. St. Chrysostom also says: "Having said, 'For if God did not spare the natural branches,' the Apostle did not continue, 'neither will He spare you.'
Instead he says, 'lest He also not spare you.' Thus, excluding harsh expressions from his discourse, he makes the believer careful, draws the Jews, and humbles the Gentiles."