Jeremiah 31:33 → Psalm 40:8

In Psalm 40:8 the willing Davidic king professes "I delight to do Your will, O my God; Your law is within my heart" — tôrātəkā bətôk mēʿāy, the Torah lodged within his inward parts. Read against the Deuteronomic charge that the king write the law on his own scroll and keep it before him (Deut 17:18-20), this is the king in whom obedience has become interior delight, not external constraint. Jeremiah 31:33, voicing the new-covenant promise, speaks in the same law-within idiom and extends it to the whole people — "I will put My law in their minds and inscribe it on their hearts" (tôrāh … ʿal-libbām) — so that what was true of the one anointed king becomes the promised reality for the whole covenant people. The shared idiom (Torah internalized within the person, heart/inward parts as the locus of the law) — one Psalm 37:31 and Isaiah 51:7 also carry — is not a quotation; the relation is Redemptive-Historical Progression along the Law-on-the-heart longitudinal thread — David's solitary internalized-Torah profession advancing to the corporate new-covenant fulfillment — not typology, since no type-institution is escalated, only an earlier instance of the thread expanded. The road to Christ runs through both: He is the true willing King in whom "a body You prepared for Me" (Heb 10:5, quoting Ps 40:6 — the anchor) and "I delight to do Your will" (Ps 40:8) coincide perfectly (Heb 10:5-9), and He is the new-covenant Mediator who, having done that will, writes the law on His people's hearts by the Spirit (Heb 10:16-17 cites Jer 31:33 right after the Psalm 40 citation). The telos is not merely a promise foreshadowed but the glory of obedience transfigured into desire: in Christ the law within the heart ceases to be a demand and becomes the deepest delight of the redeemed — Torah kept not because we must, but because, beholding Him, we finally want to.