The Pharisees were spiritually blind. Although they thought of themselves as teachers, they were unable to understand God's truth. (See: [[rc://en/ta/man/jit/figs-metaphor]])
This is a metaphor that means that if they would become pure in their inner being, then the result is that they would be pure on the outside as well. (See: [[rc://en/ta/man/jit/figs-metaphor]])