Jesus rebuked the religious rulers for missing the whole point of God's kindness and favour to the nation.  Instead of gratitude and heartfelt worship, they were arrogant and proud of their controlling positions in society.  So, Jesus unmasks them.  He likens them to unmarked graves.  In their religion, the dead were unclean, and to physically contact a grave would defile their holiness (Leviticus 21:11).  Burial grounds were best avoided in case they might unknowingly walk over a dead body.  

So, when Jesus called them a graveyard, that was a massive insult.  But He went further in saying that they were unmarked graves.  In other words, instead of helping to purify people, they spiritually contaminated them - while the people were unaware that their spiritual leaders were a danger to their relationship with God.

While most of Jesus' sayings did not cause an outward reaction, this one did.  One of the leaders realised exactly what He was saying and that it was not a compliment.  He felt insulted and said so.  But it was only the insult that truth brings to error, or which light brings to darkness.  Whenever the Lord is intent on asserting His authority in people's lives, His intervention will be a challenge because righteousness cannot come in until unrighteousness is recognised for what it is … and repented of.

While God's rebukes will insult our pride, they are never hostile to our souls but are designed out of His love for us because He wants us to repent and change (Proverbs 3:11-12).  However, if we resist His rebukes, we are fighting against God (Acts 5:38-39).  Those who lead in families, churches, business or government have a special responsibility to bring others into the truth.  They have a special responsibility to do nothing which would further contaminate their souls or lead them away from the hope of heaven through the grace of the Lord Jesus Christ (James 3:1).  So, if in some way these verses highlight such an area in your life, do not resist the Lord's rebuke but repent of harbouring darkness and darkening others.