Romans 11:20

Well; by their unbelief they were broken off, and thou standest by thy faith. Be not highminded, but fear: "Well" -'Quite right' (NASV). Some of the natural branches were broken off. But it wasn't. matter of superiority vs. inferiority. (You see, this is the very mistake that many of the Jewish people had made, i.e. we are superior to non-Jewish races.)

"by their unbelief they were broken off" -They weren't broken off because God was faithless, but because they were faithless. God has never promised to save the person that persists in unbelief. (Romans 1:17)

"thou standest by thy faith" -'you owe your position to your faith' (Mof), 'and you only stand through your faith' (Wey), 'by faith you hold your place' (NEB) (Romans 5:2)

'Why were they cut off? Because of unbelief. And if. spirit of pride leads the new graft--the Gentile (believing)...to forget its reliance on divine grace and exchange faith in God for self-confidence, it will suffer the same fate as the old branches.'

'There is nothing in Gentile flesh or personality that is the ground of their acceptance with God. Jews lost blessing because of arrogance and pride. Gentiles can do the same. Faith is something everyone can have. Gentiles have no monopoly on it. Be reverent, not arrogant.' (McGuiggan p. 329)

"Be not highminded" -5309. hupselophroneo hoop-say-lo-fron-eh'-o; from. compound of 5308 and 5424; to be lofty in mind, i.e. arrogant: -be highminded. 'Stop thinking high (proud) thoughts' (Robertson p. 397)

As so many places in the Bible, we find that pride and self-reliance often form the root of sin and unbelief. (Romans 1:22) (Check especially the book of Proverbs. (Romans 16:18; Romans 17:19; Romans 29:23)

"but fear" -no 'once saved always saved here'!