A.T. Robertson 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)

If they continue not in their unbelief (εαν μη επιμενωσ τη απιστια). Third class condition with the same verb used in verse 22 of the Gentile. Locative case of απιστια here (same form as the instrumental in verse 20).

For God is able (δυνατος γαρ εστιν ο θεος). See this use of δυνατος εστιν in 4:21 rather than δυνατα. This is the χρυξ of the whole matter. God is able.