Every valley shall be exalted, and every mountain and hill shall be made low: and the crooked shall be made straight, and the rough places plain:
Every valley shall be exalted ... Eastern monarchs send heralds before them in a journey, to clear away obstacles, make causeways over valleys, and level hills. So John's duty was to bring back the people to obedience to the law, and to remove all self-confidence, pride in national privileges, hypocrisy, and irreligion, so that they should be ready for His coming (Malachi 4:6; Luke 1:17).
The crooked - declivities.
Shall be made straight. Contrast Ecclesiastes 1:15, "That which is crooked cannot be funds straight:" but "the things which are impossible with men are possible with God" (Luke 18:27).