Who can explain what this sentence means?

This is an inverted sentence. If you can't see the inner filial piety, but show the outer filial piety, it means that the person who is kind to you may have a knife hidden in his smile. Biting without mercy means that the person who hides the knife in your smile will hurt you deeply. Needless to say, the second sentence is to advise you to be careful. It means that when you know that this person has hidden the knife in your smile, it is too late. This seems to be written by a fortune teller. I am a Christian, but I can tell you. Amen.