John Calvin Commentary Romans 11:21

"for if God spared not the natural branches, neither will he spare thee." — Romans 11:21 (ASV)

For if God has not spared the natural branches, etc. This is a most powerful reason to suppress all self-confidence; for the rejection of the Jews should never enter our minds without striking and shaking us with dread. For what ruined them, if not that through complacent dependence on the status they had achieved, they despised what God had appointed? They were not spared, though they were natural branches; what then will be done to us, who are the wild olive and aliens, if we become excessively arrogant? But this thought, while it leads us to distrust ourselves, also tends to make us cling more firmly and steadfastly to the goodness of God.

And here again it becomes clearer that the discourse is addressed generally to the body of the Gentiles; for the excision of which he speaks could not apply to individuals, whose election is unchangeable, based on the eternal purpose of God. Paul therefore declares to the Gentiles that if they exulted over the Jews, a due penalty for their pride would await them; for God will again reconcile to himself the first people whom he has divorced.