“to turn the hearts of fathers to their children.” This is quoted from Malachi 4:6. When the angel appeared to Zechariah and said that the boy would be “great in the sight of the Lord” (Luke 1:15), that was wonderful but not overly revealing as to who the child would really be. But all that changed in Luke 1:16-17. The angel started describing John’s calling and ministry in Old Testament terms that Zechariah, a well-educated and knowledgeable priest, would have known. The angel started using vocabulary and phrases from the Old Testament that revealed that John would be the forerunner to the Messiah.
The prophecies of the coming Messiah had been given for 4,000 years, starting with Genesis 3:15. Much later in the Old Testament, Scripture foretold there would be a messenger before the Messiah who would prepare the way for him (cp. Isa. 40:3-5; Mal. 3:1). In order to appreciate how important this forerunner was, we must remember that at that time no one knew there would be two “comings” of the Messiah: one when he died and one when he conquered. Everyone thought that when the Messiah came he would conquer the earth and set up his kingdom. This was why when Jesus told the Apostles that he would die that Peter said that would not happen to him (Matt. 16:22), and why the people said the Christ would live forever (John 12:34). Thus, for the angel to indicate that John would be the forerunner to the Messiah meant to Zechariah that the Messiah, and the wonderful kingdom he would set up on earth, were coming very soon.
The angel’s reference to the “spirit and power of Elijah” was a reference to Malachi 4:5, and thus was more evidence that John would come right before the Messiah. The Jews knew that Elijah was to come before the Messiah (Matt. 17:10), so they asked John if he were Elijah, to which he answered “No” (John 1:21). While that seems surprising at first, the reason that John said “no” was not because he was not the Elijah of Malachi 4:5, but because the people of the time so badly misunderstood Malachi’s prophecy about Elijah that John did not fulfill their incorrect expectations.
The rabbis believed there would be “a return of Elijah in person to prepare the Messianic kingdom” and they thought that perhaps in John “this rabbinic expectation was fulfilled and that the Baptist actually was Elijah returned to life. In this sense the Baptist utters his denial…‘I am not’” (R.C.H. Lenski, The Interpretation of St. John). So the Jews were looking for the real Elijah to be raised from the dead, and John was not that Elijah; he was not Elijah raised from the dead. However, John was the person meant in Malachi 4:5 and represented as “Elijah” by the figure of speech antonomasia (“name change,” see commentary on John 1:21). Furthermore, Jesus understood that John was “Elijah” (Matt. 11:14; 17:10-13), and Zechariah would have understood that too at some level, and understood, and no doubt was astounded to learn, that his son would be the “Elijah” of Malachi and the forerunner to the Messiah.
It is likely that this powerful and unexpected revelation, that John would be the promised forerunner to the Messiah, was at least in part why Zechariah asked the angel for a sign that these things would happen (Luke 1:18).
“their children.” Cp. NIV. In the text there is no word for “their,” however, the possessive is implied.
“good sense.” The Greek is phronesis (#5428 φρόνησις). This is not the Greek, sophia, wisdom, but rather “a word for practical intelligence” (Robertson, Word Studies).