Elijah

The angel is telling Zacharias that John the Baptist will “go before” (Luke 1:17) whom?
“God” (Luke 1:16).

Before whom did John the Baptist go?
Jesus.

So what did the angel express?
Jesus is God.

Why does the angel bring up “Elijah” (Luke 1:17)?
Elijah was the prophet whom God used to turn Israel back to Himself after the nation reached arguably its lowest point, when all but “seven thousand in Israel” (1 Kings 19:18) worshiped the idol Baal after having been led astray by the wicked king Ahab and his even more wicked queen Jezebel. At the confrontation on Mount Carmel, where Elijah squared off against 450 prophets of Baal and which led to their annihilation, “the fire of the LORD fell and consumed the burnt sacrifice” (1 Kings 18:38) and everything in its immediate vicinity after Elijah cried out, as the people of Israel watched, “Answer me, O LORD, answer me, that this people may know that You are the LORD God, and that You have turned their hearts back to You again” (1 Kings 18:37). God eventually closed the Old Testament with a promise to send a prophet like Elijah as His forerunner to “turn the hearts of the fathers to the children, and the hearts of the children to their fathers” (Malachi 4:6). Like Elijah, John the Baptist will be used “to turn the hearts of the fathers to the children” (Luke 1:17) and to “turn many of the sons of Israel to the Lord their God” (Luke 1:16), “to make ready a people prepared for the Lord.”