Earthy kings must have a way to distinguish between those who will be loyal to theire earthly kingdom. This is necessary, among other reasons, so the earthly ruler can know to whom he can delegate power. The most common earthly means of making this determination is by citizenship. And the most common tests for citizenship are birth and an oath. It was true in first century when Jesus was teaching about the kingdom and it is true today.
For example, if you are born in the United States you are automatically a citizen. If you weren’t born in the United States you can still become a citizen by confessing your loyalty by means of an oath. Initially, both may seem arbitrary, but they are actually good tests. Birth has traditionally been a basis for citizenship because it rests on the assumption that those who are dependant upon the land of a kingdom for their survival have an interest in protecting it. Oaths are an indication of what is in a person’s heart. Sure they can be faked, but until governments learn to read minds oaths are the next best thing.
Interestingly, one becomes a citizen of the kingdom of God by both a birth and an oath. First, one must be born again. John 3:5 (“Unless one is born of water and Spirit, he cannot enter into the kingdom of God.”). Second, one must give an oath. Romans 10:9 (“…for if you confess with you mouth Jesus as Lord, and believe in your heart that God raised him from the dead, you shall be saved…”).
Just as in earthly kingdoms, birth and oath are an indicia of loyalty to the kingdom of God. That demonstrated loyalty paves the way for King Jesus to share his rule with his citizens by delegating authority to them. But more on that later. GS