John Calvin Commentary Romans 11:23

"And they also, if they continue not in their unbelief, shall be grafted in: for God is able to graft them in again." — Romans 11:23 (ASV)

For God is able, etc. Unconvincing would this argument be to the profane; for although they may concede power to God, yet as they view it at a distance—confined, as it were, to heaven—they for the most part rob it of its effect. But as the faithful, whenever they hear God’s power named, perceive it as presently active, he considered this reason sufficient to impress their minds.

We may add that he assumes this as an acknowledged axiom—that God had punished the unbelief of His people in such a way as not to forget His mercy, consistent with what He had done before, having often restored the Jews after He had apparently banished them from His kingdom. And he shows at the same time by the comparison how much easier it would be to reverse the present state of things than it was to introduce it; that is, how much easier it would be for the natural branches, if they were again put in the place from which they had been cut off, to draw nourishment from their own root, than for the wild and the unfruitful to draw it from a foreign stock. For such is the comparison made between the Jews and the Gentiles.