A.T. Robertson Commentary Luke 1:17

"And he shall go before his face in the spirit and power of Elijah, to turn the hearts of the fathers to the children, and the disobedient [to walk] in the wisdom of the just; to make ready for the Lord a people prepared [for him]." — Luke 1:17 (ASV)

Before his face (ενωπιον αυτου). Not in the ancient Greek, but common in the papyri as in LXX and N.T. It is a vernacular Koine word, adverb used as preposition from adjective ενωπιος, and that from ο εν ωπ ων (the one who is in sight).

Autou here seems to be "the Lord their God" in verse 16 since the Messiah has not yet been mentioned, though he was to be actually the Forerunner of the Messiah.

In the spirit and power of Elijah (εν πνευματ κα δυναμε Ελεια). See Isa 40:1-11; Malachi 3:1–5. John will deny that he is actually Elijah in person, as they expected (John 1:21), but Jesus will call him Elijah in spirit (Matthew 17:12).

Hearts of fathers (καρδιας πατερων). Paternal love had died out. This is one of the first results of conversion, the revival of love in the home.

Wisdom (φρονησε). Not σοφια, but a word for practical intelligence.

Prepared (κατεσκευασμενον). Perfect passive participle, state of readiness for Christ. This John did. This is a marvellous forecast of the character and career of John the Baptist, one that should have caught the faith of Zacharias.