Deuteronomy 17:18

18.And it shall be, when he sitteth upon the throne. It would not be enough to correct their errors unless kings were also instructed in the fear of God, and properly taught their duty; now, therefore, a system of discipline is added, whereby it was profitable for them to be grounded in the study of religion and justice, viz., that they should take the Law from the priests and Levites, which was to be the rule of all their actions. Because the demonstrative pronoun is used, (73) some think that only the book of Deuteronomy is referred to, but without good reason. I make no doubt but that the whole sum of doctrine is included, which is delivered both here and in Exodus and Leviticus. But although it was without exception to be common to all, yet in order that kings might be more assiduously attentive in reading it, God would have a copy peculiarly dedicated to their use by the priests and Levites, and given into their hands in a solemn ceremony; that kings might know that they required greater wisdom and counsel for ruling the people than private persons. When, therefore, the priests and Levites presented them with this book, it was as if God deposited this treasure with the king. He then enjoins that they should exercise themselves in the doctrine of the Law through the whole course of their lives, because kings are usually supplied with books only out of ostentation and pomp, and when they have tasted of what is taught in them, straightway grow tired and cease to read them. Finally, the object of their reading is subjoined: first of all, in general, that they may learn to fear God and keep His statutes; and, secondly, lest, being lifted up with pride and vanity, they should despise and oppress their brethren. And the word brethren is used designedly, lest they should imagine that the law of brotherhood was abolished, because they were set over the whole people; but rather that they should study to cherish all as members (of themselves.) Again, it is afterwards repeated, lest they should “turn aside to the right hand or the left;” because, when men have much liberty of action, their lusts can never be sufficiently restrained. But, lest it should be grievous to them to be thus reduced to order, finally God reminds them that this moderation would be useful to them, for that they thus would prolong their reigns; whereas the tyranny of kings is often their destruction; as the Lacedemonian king replied, when his wife was annoyed that the Ephori were appointed to restrain him, “that he should indeed leave less power to his children, but that it would be the more lasting. (74) But, here a long succession is promised by God’s favor, if they were willing to guide themselves aright.