John Gill

Every valley shall be exalted, and every mountain and hill shall be made low,.... Which is not to be understood literally, but, as Kimchi says, parabolically and mystically: the meaning is, that in consequence of John's ministry, and our Lord's coming, such who were depressed and bowed down with the guilt of sin, and were low and humble in their own eyes, should be raised up and comforted; and that such who were elated with themselves, and their own righteousness, should be humbled; their pride and haughtiness should be brought down, and they treated with neglect and contempt, while great notice was taken of lowly minded ones; see Luk 14:11 and Luk 18:14, and the crooked shall be straight and the rough places plain; what before was dark and intricate in prophecy should now become clear; and such doctrines as were not so well understood should now become plain and easy.