Albert Barnes on Romans 11:23

And they also. The Jews.

If they bide not, etc. If they do not continue in wilful obstinacy and rejection of the Messiah. As their unbelief was the sole cause of their rejection, so, if that be removed, they may be again restored to the Divine favour.

For God is able, etc. He has

(1.) power to restore them; to bring them back, and replace them in his favour.

(2.) He has not bound himself utterly to reject them, and for ever to exclude them. In this way the apostle reaches his purpose, which was to show them that God had not cast away his people, or finally rejected the Jewish nation, Rom 11:1,2. That God has this power, the apostle proceeds to show in the next verse.

(w) "if they bide" 2Cor 3:16