Jesus was talking about their false teachings.
Sometimes we have to admit we are wrong.
That means we once believed something that we no longer believe. It also means we are capable of being deceived. This scares us. We end up guarding against deception by not listening to anything new.
We can guard against deception by doing what Second Timothy says, “Be diligent to present yourself approved to God as a workman who does not need to be ashamed, accurately handling the word of truth” 2 Tim 2:15-16 (NASU).
Trust God to show you the truth. Not man.