He alone could satisfy God's justice. The idea of blood being necessary in the forgiveness of sin is repugnent to many. What a man believes about the blood of Christ will be founded upon his belief in the literal occurrence of the sin of Adam in the Garden of Eden. They ate and the penalty was death. This is why it was absolutely necessary for Christ to die in order to be substituted for the penalty that man deserved.
Man thinks of being clean as something that happens to the outside of the body.
Being clean by Biblical standards is being free from sin. He has the right to ask that we forgive others. It is actually His place to mete out any justice for sins committed. Whenever we feel that we just cannot forgive someone for what they have done to us, it is good to contemplate what He undeservedly suffered just to preserve our relationship with the Father. For consider him that endured such contradiction of sinners against himself, lest ye be wearied and faint in your minds.