Romans 11:23

If they abide not in unbelief - So, we find that their rejection took place in consequence of their wilful obstinacy: and, that they may return into the fold, the door of which still stands open.

For God is able to graft them in again - Fallen as they are and degraded, God can, in the course of his providence and mercy, restore them to all their forfeited privileges; and this will take place if they abide not in unbelief: which intimates that God has furnished them with all the power and means necessary for faith, and that they may believe on the Lord Jesus whenever they will. The veil now continues on their heart; but it is not a veil which God has spread there, but a veil occasioned by their own voluntary and obstinate unbelief: and, when they shall turn to the Lord, (Jesus), the veil shall be taken away. See what the apostle has said, 2 Corinthians 3:6-18.

And they also - The Jews.

If they bide not ... - If they do not continue in willful 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 favor.

For God is able ... - He has,

(1) Power to restore them, to bring them back and replace them in his favor.

(2) he has not bound himself utterly to reject them, and forever 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; Romans 11:1-2. That God has this power, the apostle proceeds to show in the next verse.