Have you noticed every superhero has a symbol on their chest? Superman has the S, Spider-Man has the spider, Batman has the bat. That symbol isn't just decoration: it says who the hero is and what they stand for!
Now imagine a boy who goes around all day saying: "I'm a good kid, I'm a good kid," but at recess he hits his friend and takes his snack. That boy is saying one thing but doing another. The symbol on his chest is lying!
The Bible says wisdom needs to be kept in the heart (so we truly believe it) and fastened around the neck (so everyone can see it in our actions). Just knowing isn't enough. You have to live it!
The true guardian of the heart acts right even when nobody is watching.