Shared ground
Deuteronomy 17:18–20 places Israel’s king under the same covenant instruction as everyone else. The king is not treated as the source of “the law,” but as someone who must submit to it. The text’s concrete requirement is striking: once installed, the king must produce a personal written copy of “this law,” using the priests’ kept text as the authorized source (v.18). He must keep it with him and read it for life (v.19).
The passage also states why: regular reading is meant to shape the king’s inner posture (“learn to fear Yahweh”) and outward practice (“keep… and do” the commands and statutes) (v.19). The law functions as a check on royal arrogance and drift, so the king does not treat fellow Israelites as beneath him or wander from the command (v.20).
Where interpretation differs
Two main questions come up.
First, how much “this law” includes. Some think it means the whole book of Deuteronomy (or a large covenant document). Others think it refers more narrowly to the king-related rules just given, or to a core legal collection within Deuteronomy. The passage itself does not specify the size of the material.
Second, what “so that he may prolong his days… he and his children” means (v.20). Some read it as a fairly direct promise of dynasty stability tied to obedience. Others read it as the stated ideal outcome of covenant-faithful kingship, without claiming it guarantees long life in every individual case.
Why the disagreement exists
The wording “this law” can refer to different scopes in Deuteronomy, depending on context, and v.18 does not define its boundaries. Likewise, “prolong his days” is stated as an intended result, but the text does not clarify whether it works like an automatic guarantee or like a general covenant pattern that can have exceptions and complications.
What this passage clearly contributes
Explicitly, it teaches that royal authority in Israel is meant to be accountable to a publicly recognized, priest-guarded written standard (vv.18–19). It also links literacy-like attentiveness (copying, keeping, reading) with moral and political outcomes: reverence for Yahweh, practical obedience, humility toward fellow Israelites, and steadiness rather than deviation (vv.19–20). The passage presents the king’s long-term stability as connected to this posture of submission to Yahweh’s instruction, extending to his descendants “in the midst of Israel” (v.20).