Nephi uses this quote to proclaim Christ's greatness:
Behold, God is my salvation; I will trust, and not be afraid; for the Lord Jehovah is my strength and my song; he also has become my salvation. Therefore, with joy shall ye draw water out of the wells of salvation. And in that day shall ye say: Praise the Lord, call upon his name, declare his doings among the people, make mention that his name is exalted. Sing unto the Lord; for he hath done excellent things; this is known in all the earth. Cry out and shout, thou inhabitant of Zion; for great is the Holy One of Israel in the midst of thee. (2 Nephi 22:2-6.)
There is no greater witness of Christ than Nephi. He proclaimed Christ's grace, mercy and kindness. He testified of Christ's condescension. He bore testimony of Him hundreds of years before His birth. In detail, substance and interpretive meaning, Nephi's witness of Christ was equal to the testimony of any of the writers of the four Gospels. As he wrote in 2 Nephi 25:13: "Wherefore, my soul delighteth to prophesy concerning him, for I have seen his day, and my heart doth magnify his holy name." In significant measure, Nephi's Isaiah is a profound testimony of the Lord by an eye-witness who saw Him before His birth. It is a joyous additional testimony and witness of Christ's role as Messiah, the hope of ages, the redeemer of mankind, and Nephi's Lord. There is no greater human witness of the Lord's status and role than Nephi.
Although Nephi may have been forbidden to write his own words of Christ's future mortal ministry, latter-day accomplishments, Second Coming and Millennial reign, Nephi found words with which to testify to us of all these things. He did so by adopting Isaiah's words as his own. There are few testimonies ever recorded as powerful as we find in Nephi's Isaiah.