Because if He were a King in name only, I would rather go back to the chocolate factory. I would remain a Christian, but I would know that my religion was only a set of principles, excellent and to be followed, but hardly demanding devotion. Suppose on the other hand that I were to discover God to be a Person, in the sense that He communicated and cared and loved and led. That was something quite different. That was the kind of King I would follow into any battle.
