Albert Barnes Commentary Romans 11:21

"for if God spared not the natural branches, neither will he spare thee." — Romans 11:21 (ASV)

For if God, etc. If God did not refrain from rejecting the Jews who became unbelievers, certainly He will not refrain from rejecting you in the same circumstances. It can be assumed that He will be just as ready to reject the ingrafted branches as to cast off those that belonged to the parent stock. The situation of the Gentiles does not afford them any security greater than that of the rejected Jew.