For if God ... - 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 cast 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.
Related Commentaries of Romans 11:21
Romans 11:21
21 For if God spared not the natural branches, take heed lest he also spare not thee.