Albert Barnes on Romans 11:21

For if God, etc. If God did not refrain from rejecting the Jews Who became unbelievers, assuredly he will not refrain from rejecting you in the same circumstances. It may be supposed that he will be quite as ready to reject the ingrafted branches, as to east off those which belonged to the parent stock. The situation of the Gentiles is not such as to give them any security over the condition of the rejected Jew.