The King of Israel was intended to be the personification of the Law of God, and this was exemplified by King David, and is expected of the ultimate Son of David. This tradition finds expression in Jesus Christ, in the New Testament, and up to the present day in Patristic writings and hymnography.
Notice in Deuteronomy 17 several important points:
1.) The king that Israel establishes over themselves is to be an Israelite, not a foreigner.
2.) The king is to be someone whom God has chosen (this indicates prophetic involvement).
3.) The king is to avoid some very specific actions (all of which were failures of Solomon).
4.) Very interestingly the king is to do the following (17.18-20 RSV):
[H]e shall write for himself in a book a copy of this law, from that which is in the charge of the Levitical priests; and it shall be with him, and he shall read in it all the days of his life, that he may learn to fear the LORD his God, by keeping all the words of this law and these statutes, and doing them; that his heart may not be lifted up above his brethren, and that he may not turn aside from the commandment, either to the right hand or to the left; so that he may continue long in his kingdom, he and his children, in Israel.
Thus the king of Israel was to have internalized the Law, and to have been a living exemplar of it. As he would be the ultimate legal authority in Israel, his own person would be seen as, in a way, the Law itself. He effectively was “the law” as the ultimate legal authority, but the goal described here is greater: the personification of the Law of God itself. This seems to have been the Prophetic ideal for all of the kings of Israel and Judah, and which most failed, of course.
This concept manifests itself in narrative in the Book of Judges, particularly in 17.6, 18.1, 19.1, introducing three particularly awful events dealing with a breakdown of law and order in Israel, and in the final verse of the entire book, which may be seen as a commentary on, at the very least, this latter section of Judges, if not on the entire Book of Judges itself:
In those days there was no king in Israel; every man did what was right in his own eyes.
While 18.1 and 19.1 do not include the phrase “every man did what was right in his own eyes,” it is clearly implied. Here we see the king held up as an embodiment of order, again, as embodying the Law of God for the people. A righteous king would have prevented these occurrences. An unrighteous king is not in view here, but rather a king, living and administering God’s Law, who prevents such sinful and horrible events as those of chapters 17 through 22 from occurring.
Coming to the establishment of the kingship over Israel, there is the selection through the Prophet Samuel of Saul (who is a failure, doing nothing that he is instructed to do by the Lord through the Prophet Samuel). God, however, withdraws His favor from Saul, and trsansfers the kingship over Israel to David, “a man after His own heart” (1 Sam 13.14), establishing his throne forever, with the explicit statement that He will never take it away from David as He has from Saul (2 Sam 7) despite punishment. Throughout the latter part of the First Book of Samuel and the Second Book of Samuel, David repeatedly acts in accordance with God’s Law when others do not, including Saul, Nabal, and Absalom most obviously. Despite his sins, his standing with God remains good due to his repentance, so that David remains the ideal king of Israel, against whom others are judged. He fulfilled the role of the expected king of Israel to embody the Law itself for the nation. And this Law is embodied not simply through obedience of the various injunctions found in the Pentateuch, but is in fact shown in David to be an attitude and behavior that is repentant and focused on a right relationship with God so that even the unwritten intent of the Law is constantly (if not permanently) in view in David’s life and actions, so that he comes to be known as a man after God’s own heart.
It is precisely this exemplary embodiment of the Law in David that then becomes, through the ages, transferred to the expected ultimate Son of David, the Anointed, the Messiah. He will be everything his father David was and more. See especially Isaiah 7 and 42, and Jeremiah 31, in the latter of which the embodiment of the Law is extended to all the people of Israel. This is an extremely important development in the understanding of the Law, one which bears unexpected fruit.
This full development of the Law as resting particularly in the King, and this most particularly exemplified first in David and then in the expected, ideal, ultimate Son of David, comes to expression in the New Testament as well. Much is made of the dichotomy of Law versus freedom in the writings of Paul, for instance, without careful attention to the extreme respect and hedging about of utter rejection of the Law. The language Paul and the other Apostolic writers of the New Testament use instead is based in one of the following: 1.) Jesus, the Anointed, the Sond of David, fulfills the Law, bringing to it everything it was lacking, bringing it to completion, connoting a superiority of Jesus to the Law; 2.) The Law of Jesus incorporates and supersedes the old Law, again connoting a superior authority embodied in Jesus, who can prescribe a new context for the old Law; 3.) related to the second point, but with more emphasis: Jesus is the Lawgiver, the same one anciently and currently, Who gave Israel the Law at Sinai and gives a new Law now to Christians. In all of these, Christ is set above the Law as described in the Books of Moses, but He is a living Law: He is an embodiment of the Law. As the Son of David par excellence, the ancient tradition of the Law’s embodiment in the King has come to rest in Jesus Christ.
As has been traditionally understood, Pentecost was the day on which the Law was given at Mount Sinai. In the Church, Pentecost is known for the descent of the Holy Spirit onto the earliest Christians, as described in the Book of Acts. In the hymnography of the Eastern Church, this dual import of the day is not lost: the latter is seen as superior to the former meaning, just as the above-mentioned Jeremiah 31 describes that this is precisely the way that the Law is written upon the hearts of Israel: through the Holy Spirit of God. This is one example taken from Matins of Pentecost Sunday, from the Canon, Ode Eight (Holy Transfiguration Monastery translation):
The bush that was unconsumed by fire on Sinai spake unto the tardiloquent and inarticulate Moses, and made God known unto him; and zeal for God showed forth the three Children who chanted hymns to be unconsumed by fire. O all ye His works, praise ye the Lord and supremely exalt Him unto all the ages.
When the vivifying, violent wind of the All-Holy Spirit came from on high, resounding unto the fishermen in the form of fiery tongues, they spake eloquently concerning the mighty deeds of God. O all ye works, praise ye the Lord and supremely exalt Him unto all the ages.
Ye that ascend not that untouchable mountain, nor fear the awesome fire, let us stand on Mount Sion, in the city of the living God, and now form one choir with the Spirit-bearing disciples. O all ye works, praise ye the Lord, and supremely exalt Him unto all the ages.
This is the ancient Christian perspective that the latter Pentecost fulfills, completes and enhances and supersedes the earlier, in that the people now have the Law written in their hearts through the direct action of God though the Holy Spirit. This is one reason that the ancient Holy Sion church on Mount Sion (the western hill of Jerusalem) was such a major attraction. The “upper room” in which this occurred, the new Christian Sinai, was incorporated into the church complex, and was thronged with pilgrims.
This is another piece of the puzzle. There was a body of traditions associated in popular and perhaps scholarly imagination with the coming Son of David prior to the advent of Jesus Christ. Various of these appear in the Gospel accounts, typically indicating that the people believed one thing, while Jesus, the Son of David, had another thing in mind. In some cases, however, there was overlap, but even then the fulfillment in Jesus is seen to have taken the traditional conception in an unexpected direction. These various traditions, as I describe here, were rooted in the Old Testament texts, and are found described in later writings more clearly. By the time Jesus appears, as described in the New Testament, there was a very well-developed body of traditional expectations for what the Son of David would be: healer, king, conqueror, etc. But it is perhaps in the aspects of His mission that did not align with the ancient expectations that the most extraordinary revelations are seen to occur, His role as Lawgiver being perhaps the most significant. This too, appears to have been expected. And though the evidence is somewhat diffuse, somewhat tenuous in the Old Testament writings, it is still there. After all, who describes in detail those things which are taken for granted? A king is always the law wherever he is. In this case, it is a very particular King embodying a very particular Law: the Son of David embodying the Law of God.