Romans 11:20

‘Well, by their unbelief they were broken off, and you stand by your faith. Do not be highminded, but fear,'

So Paul reminds them that the Jewish branches were broken off because of their unbelief, whilst they themselves have been engrafted in, by faith in the Messiah. Thus they should not be highminded, seeing themselves as something special by their own merits, but should rather recognise that they owe it all to Christ. Indeed they should be on their guard, ‘working out' their salvation with fear and trembling as God works it within them (Philippians 2:12), recognising that it is only through faith in God's goodness and Christ's sacrifice that they enjoy the position that they are in.

We see in these verses the clear interconnection between the major theme of the salvation of the Gentiles through being incorporated in the olive tree, and the secondary theme of behaving in a godly manner towards the unbelieving Jews.