Question: How is the Book of Mormon another testament of Jesus Christ?
Answer: The divinity of Jesus Christ is powerfully proclaimed by prophets in the Old and New Testaments and in the Book of Mormon.
The Book of Mormon record, kept by an ancient people living from 600 BC to AD 421, has come forth as a powerful second witness of Christ.
From the time that Lehi left the land of Jerusalem in 600 BC until Moroni deposited the plates in the Hill Cumorah in AD 421, this account remained in the possession of Lehi and his posterity. Prophets from this lineage wrote of their knowledge, faith, and testimony of the Beloved Son of God.
The Book of Mormon and the Holy Bible are witnesses of the divine mission of Jesus Christ. These sacred records speak of the divinity of the Son of God, His ministry, and His holy teachings.
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 33.