What is the meaning of this phrase?

"When the fox preaches, look to the geese." Can anyone explain this German proverb?

It is about keeping an awareness of hidden agendas like when I get calls from telemarketers wanting to "help" me refinance my mortgage.

The fox has but one agenda which is to eat the geese.
Your proverb states that if you let your guard down then Mr. Fox will do what comes naturally so stay alert.