Matthew 3:3

For this is he that was spoken of
These are not the words of the Baptist himself, as in ( John 1:23 ) but of the Evangelist, who cites and applies to John a passage in the Prophet Isaiah, ( Isaiah 40:3 ) and that very pertinently, since that "chapter" is a prophecy of the Messiah. The consolations spoken of in ( Isaiah 40:3 ) , were to be in the days of the king Messiah, as a writer of note ^F25 among the Jews observes. The Messiah is more expressly prophesied of in ( Isaiah 40:9-11 ) as one that should appear to the joy of his people, and "come with a strong hand", vigorously prosecute his designs, faithfully perform his work, and then receive his reward; he is spoken of under the "character" of a "shepherd", who would tenderly discharge the several parts of his office as such, which character is frequently given to the Messiah in the Old Testament: now the person spoken of in ( Isaiah 40:3 ) was to be his harbinger to go before him, proclaim and make ready for his coming; and what is said of him agrees entirely with John the Baptist, as the character given of him,

the voice of one crying,
(Bowntov) , lowing like an ox; which expresses the austerity of the man, the roughness of his voice, the severity of his language; that he called aloud and spoke out, openly, publicly, and freely; and that he delivered himself in preaching with a great deal of zeal and fervency. The place where he preached was "in the wilderness", that is, of Judea, where he is said before, in ( Matthew 3:1 ) to come preaching. The doctrine he preached was,

prepare ye the way of the Lord, make his paths straight,
which is best explained by what is said before, in ( Matthew 3:2 )

repent ye, for the kingdom of heaven is at hand.
The Lord whom ye have sought, the Messiah whom you have expected, is just coming, he will quickly appear; prepare to meet him by repentance, and receive him by faith, relinquish your former notions and principles, correct your errors, and amend your lives, remove all out of the way which may be offensive to him. The allusion is to a great personage being about to make his public appearance or entrance; when a harbinger goes before him, orders the way to be cleared, all impediments to be removed, and everything got ready for the reception of him.