Luke 11:44

“Woe to you! for you are as the tombs which are not visible, and the men who walk over them are not aware of it.”

The third woe is that they cause others to stumble. To touch a tomb or grave rendered a person ‘unclean' and meant a seven day period of cleansing. It was therefore important that tombs were clearly visible. Indeed some time before the Passover all graves in Palestine would be painted white so that Passover pilgrims might not be accidentally defiled and thus unable to eat the Passover. So for someone to be like an unmarked tomb was for him to be a catastrophe to people.

And the Pharisees were like unmarked graves, for they did not warn people away from what was truly defiling, the attitudes of the heart. Thus they encouraged people to think that all was well with them when in fact it was far from well.

We must not be unfair to the Pharisees. The purpose of their multitude of extra ‘laws' was in order to help people to know what they should and should not do. Where they failed (and failure is inevitable with too many rules and regulations, for people will then begin to look for loopholes, and will ignore the more important attitude that should lie behind their observance) was in that, by doing so, they made people feel that they were satisfying God by what they did to such an extent that they could therefore do what they liked with the remainder of their lives. They bred hypocrites, people who played a part without really being what they should be.