And the glory of the LORD shall be revealed, and all flesh shall see it together: for the mouth of the LORD hath spoken it.
All flesh shall see it. The Septuagint for "it" have 'the salvation of God.' So Luke 3:6 (cf. Luke 2:30, "mine eyes have seen thy salvation" - i:e., Messiah); but the evangelist probably took these words from Isaiah 52:10, "all the ends of the earth shall see the salvation of our God." The change, however, is significant from "the glory of the Lord," which is here, and which points ultimately to the coming of the Lord in glory: whereas John the Baptist's mission was to usher in His coming in grace to bring salvation.
For the mouth of the Lord hath spoken (it) - rather, 'all flesh shall see that the mouth of Yahweh hath spoken it' (Bengel).