Their practice, Jesus declares, was to wash the outside of the cup and platter, while leaving the inside contaminated with putrid filth.  Their hearts were full of ravening and wickedness, but this bothered them not because all men could see was their exterior, which had a noble appearance.  The way to true holiness, Christ informs us, is to cleanse that which is within, and then the exterior will also have a good appearance.  In other words, if we cleanse our hearts from the pollution of sin, then we will avoid sin and do righteously on the outside. 

He draws a similar picture immediately below, comparing His hypocritical enemies to sepulchers, which are ornate and beautifully decorated on the outside, but if opened up and examined, are full of the stench and corruption of death.  This is a very accurate picture of the religion of the Pharisees, and indeed of hypocrites in every generation.  They practice religion in order to win the plaudits of their fellow men, and to gain a name for righteousness which they do not truly deserve.  Thus, the Pharisees performed rituals, performed their prayers, and fasted publicly, so that they may appear righteous unto men, but in their hearts and private behavior they were full of hypocrisy and wickedness.  Their lives, if examined closely, would be seen to be full of extortion, adultery, and all manner of sinful excess.