You are like unseen graves over which people unknowingly walk. (Luke 11:44)
The Lord Jesus imprecated the hypocritical Pharisees with this mysterious insult. What exactly does it signify?
Under the Old Covenant, the Lord had divided the world into three sectors: the holy zone, the clean zone, and the unclean zone. The People of God occupied the clean land surrounding the Holy Temple. Outside: Gentile, unclean.
God has revealed Himself as the Giver of Life. Holy = living—fully eternally, vigorously living. Clean = animated by this holiness, the undying vigor that comes from God. Unclean = tending toward death.
No one and nothing unclean could have a part in any sacred ceremony. In other words, when the living touched the Source of Life, nothing pertaining to death could vitiate the communion.
One of the ways in which an Israelite would be rendered unclean would be to touch a corpse, or even a grave.
The Old Law included a provision for the purification of an Israelite rendered unclean by contact with a corpse. The procedure is an example of what the rabbis called khok, that is, a law with no apparent rational logic, which must therefore be of direct divine institution.
A perfectly pure red heifer would be slaughtered outside the city. The carcass would be burned, along with cedar wood, hyssop, and scarlet yarn. The ashes were put into a container of spring water. Anyone who had touched death could be purified by that water.
In order to maintain the purity of the water, children who had been born and reared in an area of Jerusalem known to be free of graves had to carry the water in stone bowls, sitting on top of wooden slabs placed on the backs of oxen. They traversed the Kidron Valley on an elevated causeway so as to avoid stepping on any of the graves there.
In other words: NO STEPPING ON GRAVES!!!! EVER!!!!!!!!!!
What the rabbis called khokim—laws with no reasonable explanation—we call prefigurements. Yes, these laws (which have now passed out of effect) do, in fact, make sense: they provide us with insight into the mystery of Christ.
The red heifer prefigures Christ, Who also endured His sacrificial death outside the gates of Jerusalem. His death inaugurated the sacrament of Holy Baptism, the cleansing water that frees us from eternal death.
When the Lord insulted the deluded, self-serving Pharisees, He did not just accuse them of being unclean themselves. He called them a source of uncleanness, called them hidden corpses that contaminate other Israelites without their even knowing it.
By saying this, Jesus helped us to understand the true meaning of the Old Covenant system.
The division of the cosmos into holy, clean, and unclean is fundamentally real. But the zones are not primarily in the physical world; they are found in our own souls.
We touch the unseen holy by sincere prayer, by truthfulness, by careful examinations of conscience. We defile ourselves by lies—above all, by lies we tell ourselves—and by actions that in one way or another crush the life out of ourselves or others.
And we can be purified of this uncleanness by the one, true source of purity and holiness—by Jesus Christ, Whose undying life is ministered to us in the sacraments of the New and everlasting Covenant.
Father Mark White Blog 
Interesting — Thanks!!
I liked your post. See mine on my website blog