According to Bruce R. McConkie, Isaiah (Isaiah 40:3-5), Lehi (1 Nephi 10:7-10), and Nephi (1 Nephi 11:27) all prophesied “concerning a prophet who should come before the Messiah, to prepare the way of the Lord.” As Nephi recorded: “Yea, even he should go forth and cry in the wilderness: Prepare ye the way of the Lord, and make his paths straight.” (1 Nephi 10:7-8). This Book of Mormon account pertains only to John’s ministry in the meridian of time. Isaiah, however, in his prophecy, is speaking only incidentally of the preparatory work of John and more particularly and extensively of the Second Coming when every valley shall be exalted and the Lord shall be revealed to reign personally on earth.
Matthew and Mark record John‘s true claim that he came to fulfil Isaiah’s promise that one should come in that day to prepare the Lord’s path. Luke does the same, but then continues the quotation, leaving the false inference that John claimed he was then fulfilling the glorious predictions relative to the Second Coming. But in Luke’s account, as found in the Inspired version, the Prophet inserted more than five verses which show clearly that John was claiming to be the promised forerunner of time’s meridian, and that the Isaiah quotation about the mountains being brought low and all flesh seeing the salvation of God, had reference not in the first but to the Second Coming of the Lord:
As it is written in the book of the prophet Esaias; and these are the words, saying, The voice of one crying in the wilderness, Prepare ye the way of the Lord, and make his paths straight.
For behold, and lo, he shall come, as it is written in the book of the prophets, to take away the sins of the world, and to bring salvation unto the heathen nations, to gather together those who are lost, who are of the sheepfold of Israel;
Yea, even the dispersed and afflicted; and also to prepare the way, and make possible the preaching of the gospel unto the Gentiles;
And to be a light unto all who sit in darkness, unto the uttermost parts of the earth; to bring to pass the resurrection from the dead, and to ascent up on high, to dwell on the right hand of the Father,
Until the fulness of time, and the law and the testimony shall be sealed, and the keys of the kingdom shall be delivered up again unto the Father;
To administer justice unto all; to come down in judgment upon all, and to convince all the ungodly of their ungodly deeds, which they have committed; and all this in the day that he shall come;
For it is a day of power; yea, every valley shall be filled, and every mountain and hill shall be brought low; the crooked shall be made straight, and the rough ways made smooth;
And all flesh shall see the salvation of God. (Inspired Version, Luke 3:2, 4-11)
John held the priesthood, received revelations, enjoyed the companionship of the Holy Ghost, angels ministered to him, and he was expressly commanded to stand forth as the Lord’s forerunner. (D.& C. 84:26-28) The prophet Joseph Smith declared that “John, at that time, was the only legal administrator in the affairs of the kingdom there was then on the earth. And holding the keys of power, the Jews had to obey his instructions or be damned, by their own law.” (Teachings of the Prophet Joseph Smith, p. 276).
[Bruce R. McConkie, Doctrinal New Testament Commentary, Vol. 1, pp. 112-113, 115-116]