a whited sepulchre
a whited sepulchrea hypocrite; someone who is ostensibly virtuous but inwardly corrupt. literary
This expression comes from Matthew 23:27: ‘Woe unto you…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’. A sepulchre is a room or monument, cut in rock or built of stone, in which a dead body is laid or buried.
See also: white