In the Benedictus, Zechariah joyfully recalls “the oath which [the LORD] sware to our father Abraham” (Lk 1:73). What is this oath? Readers of the NT who spend a lot of time with Paul will immediately think of “the promise, that he should be the heir of the world” (Rom 4:13, cf. Rom 4:17, Gen 12:3, 15:5, 17:5). But the next verse in Luke confounds this expectation, for Zechariah elaborates: “That he would grant unto us, that we being delivered out of the hand of our enemies might serve him without fear, in holiness and righteousness before him, all the days of our life” (Lk 1:74-75). Rather than the promise of many nations’ being blessed in Abraham, Zechariah looks to the LORD’s promise to Abraham to deliver his people from bondage so as to worship him faithfully.
Where do we find this promise? The only passage I can think of is the following: “Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years; And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance” (Gen 15:13-14). As the reference to “four hundred years” makes clear, this is a promise about Israel’s slavery in and deliverance from Egypt. But this is striking — Zechariah treats a promise that had already been spectacularly fulfilled as somehow still awaiting fulfillment. He is awaiting a new Exodus (cf. Lk 3:4-6).