Luke 3:4-6 → Isaiah 40:3-5

Luke alone among the evangelists extends the Isaiah 40 citation beyond the herald-voice of v.3 through the leveling of v.4 and the climactic v.5, rendered as "all humanity will see God's salvation" (Isa 40:5 LXX, σωτήριον). Where Matthew, Mark, and John quote only the voice-and-way clause to identify John the Baptist, Luke's editorial fulfillment formula ("as it is written in the book of the words of Isaiah the prophet") carries the reader to the universal horizon that drives Luke-Acts: the salvation revealed in Christ is for all flesh, Jew and Gentile alike. This σωτήριον language is the scriptural ground of Simeon's Nunc Dimittis (Luke 2:30-32, "a light for revelation to the Gentiles"), and it anticipates the Gentile mission of Acts. The connection is Promise-Fulfillment, not typology: Isaiah 40 is a verbal prophecy of a coming herald and a coming revelation of Yahweh's glory, and Luke declares John to be that herald and the gospel age to be the dawning of that revelation. The "way of the Lord" John prepares is the way of Jesus, so Luke quietly makes the Yahweh whose glory Isaiah promised the very Christ whom John heralds — a Yahweh-Christology embedded in the structure of the citation. The telos is not John's ministry of repentance as an end in itself but the desirability of the Lord who comes: every obstacle is flattened, every valley raised, precisely so that all humanity may see the glory of God in the face of Christ (cf. 2 Cor 3:18; 4:6). The not-yet remains — all flesh has not yet seen — but the seeing has begun, and the gospel exists to make that glory visible until it is.