The King’s natural subjects are so because the King is God’s lieutenant on earth. Lieges owe fealty by the words of God as cited by Samuel. It is God’s will that Lieges obedience is due the King. Fealty is a yoke which God hath laid upon you (Lieges).
This is straight from a King of England.
Like the current Imperial President, the kings tended to like doctrines that gave them as much power as possible.