"Woe to you, scribes and Pharisees, hypocrites! for you are like whitewashed tombs, which outwardly appear beautiful, but within they are full of dead men's bones and all uncleanness. So you also outwardly appear righteous to men, but within you are full of hypocrisy and iniquity" (Matthew 23:27-28 RSV).
You asked only about verse 28, but it is relatively meaningless without the preceding verse. This is a part of Jesus' teachings in the final week of his earthly ministry, as he sits before the Temple in Jerusalem, surrounded not only by his followers, but also scribes and Pharisees hoping to catch him teaching heresy or sedition, so they can have him arrested and brought to trial. The imagery in this passage would be particularly powerful for the latter, for a person touching a dead body was considered unclean. He is telling those representatives of the status quo that a surface appearance of piety would no longer suffice. It is who we are inside—what we believe, what we choose in the privacy of our minds—that determines whether we move into the expanded consciousness he called 'the kingdom of heaven,' or stay locked in the limited consciousness of the past.