Another purpose of the Book of Mormon is “that ye may believe the gospel of Jesus Christ, which ye shall have among you.” Mormon understands that the gospel of Jesus Christ will be in the world when the Book of Mormon comes forth, but he also understands that it may not be the pure gospel. His writing is to bring us to the true understanding of the gospel. Mormon is remembering the Lord’s declaration about the nature of his gospel:
3 Nephi 27:20-21
20 Now this is the commandment: Repent, all ye ends of the earth, and come unto me and be baptized in my name, that ye may be sanctified by the reception of the Holy Ghost, that ye may stand spotless before me at the last day.
21 Verily, verily, I say unto you, this is my gospel; and ye know the things that ye must do in my church; for the works which ye have seen me do that shall ye also do; for that which ye have seen me do even that shall ye do;