And thou, child ... - Zechariah predicts in this and the following verses the dignity, the employment, and the success of John. He declares what would be the subject of his preaching, and what his success.
Prophet of the Highest - Prophet of God; a prophet “appointed by God” to declare his will, and to prepare the way for the coming of the Messiah.
The face of the Lord - The Lord Jesus, the Messiah, that was about to appear. To go before “the face of one” is the same as to go immediately before one, or to be immediately followed by another.
To prepare his ways - This is taken from Isaiah 40:3. See the Matthew 3:3 niote, and Isaiah 40:3 note.