For if God did not spare the natural branches, neither will he spare you.
Romans 11:21 ESV

Since the salvation and grafting of Gentiles is solely by faith, not because of anything they have done, some say that there is nothing they can do to lose that new status. However, Paul is delivering a clear threat that being arrogant towards the native branches that have been cut off could cause God to cut those Gentiles off again.

The concept of eternal security is so clearly refuted in these verses (and many others!) that I am amazed that anyone could ever believe it, let alone scholars and theologians.