Both oracles are eschatological covenant-promises voiced into the ruin of the monarchy, and both pivot on the same divine self-commitment: Isaiah's "I will make with you an everlasting covenant — My loving devotion promised to David" (bᵉrît ʿôlām, with the ḥasdê dāwid of 2 Sam 7 now democratized to all who "incline the ear and come," Isa 55:3) stands beside Jeremiah's "I will make a new covenant" (bᵉrît ḥădāšâ, Jer 31:31). Read as original meaning, Isaiah extends the sure mercies sworn to David to the whole listening remnant, while Jeremiah specifies the coming arrangement's distinctive form — law inscribed on the heart, immediate knowledge of God, and sins remembered no more. The link between them is genuine but light: there is no verbal dependence (Isaiah's accent is Davidic ḥesed, Jeremiah's is heart-Torah and forgiveness; the term "new" is Jeremiah's coinage alone), and neither prophet fulfills the other — so this is honestly an Echo, not a quotation or allusion, and a Longitudinal Theme parallel rather than typology (no type-antitype structure is in view) or promise-fulfillment between the two. What unites them is the one canon-wide Covenant thread: the everlasting-covenant corpus (Isa 54:10; 55:3; 59:21 alongside Jer 31:31-34 and Jer 32:40's explicit bᵉrît ʿôlām) by which God pledges an irrevocable, unilaterally-secured bond with His people. The two streams are deliberately fused only later, in Christ — Hebrews 13:20 blesses by "the blood of the eternal covenant" (Isaiah's aiōnios / Jeremiah's kainē welded into one), and Paul anchors Israel's future ingathering in the forgiveness clause (Rom 11:27). The telos: Isaiah's invitation is not "buy" but "come… without money," "that your soul may live" and "delight in the richest of foods" (55:1-3) — the everlasting covenant is offered as the satisfaction of the soul, and Jeremiah's heart-inscribed knowledge of God is what that satisfaction tastes like. Both oracles converge on Christ as the One in whom the soul finally feeds and lives — not merely a covenant foreshadowed, but the Beloved in whom the sure mercies become ours to savor forever.
