According to Isaiah 40:3,
“The voice of one crying in the wilderness: ‘Prepare the way of the LORD; Make straight in the desert A highway for our God.”
Tenth President Joseph Fielding Smith said,
“You may apply this to John if you will, and this is true…But I will go further and maintain that Joseph Smith was the messenger whom the Lord sent to prepare the way before him” (Doctrines of Salvation 1:194-195).
We must allow scripture to interpret scripture. Jesus said this was speaking about John the Baptist, not Joseph Smith. Matthew 3:1-3 says,
“In those days John the Baptist came preaching in the wilderness of Judea, and saying, “Repent, for the kingdom of heaven is at hand!” For this is he who was spoken of by the prophet Isaiah, saying: ‘The voice of one crying in the wilderness: Prepare the way of the LORD; Make His paths straight.’”