In the Sermon on the Mount, Jesus says, “Blessed are the peacemakers, for they shall be called the children of God.” To work for peace, you must have a peaceful heart. When you do, you are a child of God. But many who work for peace are not at peace. They still have anger and frustration, and their work is not really peaceful. We cannot say that they are touching the kingdom of God. To preserve peace, our hearts must be at peace with the world, with our brothers and our sisters.
Source: Seeking Peace