Looking good on the outside, but disgusting on the inside
Woe unto you, scribes and Pharisees, hypocrites! for ye are like unto whited sepulchres, which indeed appear beautiful outward, but are within full of dead men's bones, and of all uncleanness. Even so ye also outwardly appear righteous unto men, but within ye are full of hypocrisy and iniquity.
Corruption of the heart can be hidden by false behaviour - sounding good, looking good, doing good deeds; being 'good' on the outside is mere hypocrisy if the heart is deceitful and wicked.