Albert Barnes on Luke 1:76

And thou, child, &c. Zacharias 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 Isa 40:3 Mt 3:3 Isa 40:3

(u) "go before the face" Mal 3:1