A.T. Robertson Commentary Romans 11:21

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

Be not highminded (μη υψηλα φρονε). "Stop thinking high (proud) thoughts."

Of God spared not (ε γαρ ο θεος ουκ εφεισατο). It is not ε μη (unless), but the ουκ negatives the verb εφεισατο (first aorist middle indicative of φειδομα, to spare. Condition of first class.