Deuteronomy 17:14-20 https://goo.gl/3aeDFp and https://goo.gl/EqTo7c
In effect when the People of Israel arrive in the Promised Land they may set a leader above them, a King. The King will be "God Chosen" (appointed and crowned by the Church). from amongst their own people (he must not be a foreigner). When he is appointed [anointed and crowned] King he may not amass goods and wealth of huge value [gold and horses] Nor should he acquire many wives. The Priests of the Levitical Tribe [The Church] should after his appointment [coronation] set down in writing all the laws of the kingdom, which he should read and study all his life whilst he is King. He should "learn to fear the Lord his God, and keep His words and ceremonies, that are commanded in the law" and follow them religiously all his life, "that his heart be not lifted up with pride over his brethren [his own people]".
John of Salisbury Policraticus
Book IV Chapter 4
That the authority of divine law consists in the prince being subject to the justice of law.
Johannes (Sarisberiensis); Cary J Nederman ed and tr (1990). John of Salisbury: Policraticus. Cambridge University Press. pp. 35–. ISBN 978-0-521-36701-1. https://goo.gl/xXbQ4Q
Internet History Sourcebooks Project Policraticus, Book Four (selections)