Then you will say, “Branches were broken off so that I might be grafted in.”  That is true. They were broken off because of their unbelief, but you stand fast through faith. So do not become proud, but fear. (ESV)
There is always a danger in the church at large with people not taking seriously enough the things of God. An offer of God's amazing grace to us ought to fill us with a sense of awe and wonder. We should see our sinful rebellious heart and be astonished that God would show mercy and grace to such an undeserving person. His grace ought to lead us to awe and wonder, giving him glory.
Sadly, God's offer of grace is often perceived as the almighty creator looking down at us and thinking that he just could not pass up the opportunity to have us as his very own. After all, we are so wonderful. What a catch we are. This selfish kind of thinking has no basis in Scripture. There are churches that either directly or indirectly are teaching this drivel.
Paul is saying in these verses that a person who responds to the grace found in the Gospel of Jesus Christ may notice that branches were broken off so that we can be grafted in. If we are not careful we will take a selfish viewpoint of God's grace and look at the removed branches as some sort of reason to celebrate our good fortune.
Instead of seeing the branches (people of Israel who were proud and unbelieving) as reason to be proud we ought to take a much more sober look at the situation. Paul says at the end of verse 20, "Do not become proud, but fear." In other words, soberly evaluate your situation. Have you genuinely received the Gospel? If so, it will lead to you giving God the glory. You will be humble, not proud.
Have you taken the Gospel message seriously? Are you fixated on how good things are for you? Or are you in awe of God's mercy, responding in praise of his glory? A spiritual response will be a sober one.