Jamieson, Fausset and Brown on Isaiah 40:6

6. The voice--the same divine herald as in Is 40:3.

he--one of those ministers or prophets (see on Is 40:1) whose duty it was, by direction of "the voice," to "comfort the Lord's afflicted people with the promises of brighter days."

All flesh is grass--The connection is, "All human things, however goodly, are transitory: God's promises alone steadfast" (Is 40:8, 15, 17, 23, 24); this contrast was already suggested in Is 40:5, "All flesh ... the mouth of the Lord." 1Pe 1:24, 25 applies this passage distinctly to the gospel word of Messiah (compare Joh 12:24; Jas 1:10).