The citation of the New Testament quotation attributed to John (see Mark 1:7, Luke 3:16, and John 1:27) is a further indication that Lehi likely included scriptural backing for the Old Testament prophets cited in verse 5. The specific inclusion of this text was because it was unknown to Nephi, however familiar it may seem to us.
Translation analysis: Here again is a glimpse into the translation process. The voice crying in the wilderness is a reference to Esaias (with whom Lehi and his family may have been familiar - though now lost. Esaias is considered to have lived in the time of Abraham, based on D&C 84:13 “Esaias also lived in the days of Abraham, and was blessed of him--”), and the information about John the Baptist the fulfillment of prophecy. The specific manner in which the information is recorded in the Book of Mormon is clearly referenceable to the King James Version New Testament. Joseph would have seen the meaning, and made the connection to a known text which supplied that meaning. Again, the relationship of text to translation is not a direct copy of the words on the plates, but a placing of the meaning of the words into words and phrases of known and accepted meaning.