Zechariah, mute all during Elizabeth’s pregnancy with John, was finally made able to speak on the day of John’s circumcision. While it’s more dramatic to believe the first thing out of his mouth was this prophecy (and of course it makes for good movies), it’s more likely Zechariah wrote it down and shared it with lots of people, by way of thanksgiving for John’s birth. Likely John had a copy too. He is addressed in it, after all.
Let’s get to that prophecy.