John Lightfoot on Luke 3:5

[Every valley shall be filled.] The Jews have a tradition, that some such thing was done by the cloud that led Israel in the wilderness. Instead of many instances, take the Targumist upon Canticles 2:6: "There was a cloud went before them, three days' journey, to take down the hills and raise the valleys: it slew all fiery serpents in the wilderness, and all scorpions; and found out for them a fit place to lodge in."

What the meaning of the prophet in this passage was, Christians well enough understand. The Jews apply it to levelling and making the ways plain for Israel's return out of captivity: for this was the main thing they expected from the Messiah, viz. to bring back the captivity of Israel.

"R. Chanan saith, Israel shall have no need of the doctrine of Messiah the King in time to come; for it is said, To him shall the Gentiles seek (Isa 11:10), but not Israel. If so, why then is Messiah to come? and what is he to do when he doth come? He shall gather together the captivity of Israel," &c.