# Your boasting is not good

"Your boasting is bad"

# Do you not know that a little yeast leavens the whole loaf?

In this case, one person who is sinning can harm the entire Christian community. So the whole passage means: Don't you know that a little evil will affect the whole congregation? So get rid of the evil so you can live purely. Christ has been sacrificed for us. 

# Do you not know that a little yeast ... loaf?

"You know that a little yeast ... loaf." or "A little yeast ... loaf."