Symbolism: The effect of the removal of the “plain and precious parts” is apostasy. People stumble. Satan has power over them. The book symbolizes the true gospel, which could never lead to such consequences. Symbolically, only the gospel’s corruption can explain these effects. Thus, the angel both confirms the truth of the gospel and describes the apostasy from it.
Although the wording of this passage makes most sense to a modern reader as having the plain and precious parts removed after the New Testament was written, that is more information than Nephi had or would have been able to completely comprehend. The vision is accurate, but Nephi’s perspective from the historically distant side of the revelation would necessarily be different from our ability to look back on how history fit into the events of the revelation. For Nephi, the precious parts that were removed had already happened. The Book of Mormon may be seen as restoring the precious part of the Atoning Messiah.