Adam Clarke on Romans 11:20

Well; because of unbelief, etc. - This statement is all true; but then, consider, why is it that they were cast out? Was it not because of their unbelief? And you stand by faith: you were made partakers of these blessings by faith; be not high-minded; let this humble, not exalt you in your own estimation; for if the blessings were received by faith, consequently not by works; and if not by works, you have no merit; and what you have received is through the mere mercy of God. They once stood by faith; they gave place to unbelief, and fell: you stand now by faith; but it is as possible for you to be unfaithful as it was for them, and consequently you may fall under the Divine displeasure, as they have done; be not high-minded, but fear; watch over yourselves with godly jealousy.