1 Nephi 13

It is in this great vision of Nephi that we learn some truths of the Bible. One is that -

"...The book that thou beholdest is a record of the Jews, which contains the covenants of the Lord, which he hath made unto the house of Israel; and it also containeth many of the prophecies of the holy prophets; and it is a record like unto the engravings which are upon the plates of brass, save there are not so many; nevertheless, they contain the covenants of the Lord, which he hath made unto the house of Israel..." (Book of Mormon | 1 Nephi 13:23)

This suggests that there are many teachings, prophecies, and I specifically note "covenants" that are not included in our present Old Testament that was had at the time of Lehi. We also learn that the writings -

"...when it proceeded forth from the mouth of a Jew it contained the fulness of the gospel of the Lord, of whom the twelve apostles bear record; and they bear record according to the truth which is in the Lamb of God.
25 Wherefore, these things go forth from the Jews in purity unto the Gentiles, according to the truth which is in God." (Book of Mormon | 1 Nephi 13:24 - 25)

I note that "fulness" means "the quality or state of being full" suggesting the record was full of gospel covenants and teachings or the fulness of the "doctrine of Christ".

But then we learn that -

"...after they go forth by the hand of the twelve apostles of the Lamb, from the Jews unto the Gentiles, thou seest the formation of that great and abominable church, which is most abominable above all other churches; for behold, they have taken away from the gospel of the Lamb many parts which are plain and most precious; and also many covenants of the Lord have they taken away." (Book of Mormon | 1 Nephi 13:26)

What is important in my mind isn't necessarily the plain and precious parts but the covenants of the Lord. I note that there are no covenants related to entering the terrestial and telestial kingdoms. Does this not explain of those that enter the terrestial kingdom -

"These are they who are honorable men of the earth, who were blinded by the craftiness of men.
76 These are they who receive of his glory, but not of his fulness.
77 These are they who receive of the presence of the Son, but not of the fulness of the Father. (Doctrine and Covenants | Section 76:75 - 77)

These are good people that fail to take upon themselves sacred covenants, mainly because these the importance of these covenants have been taken away in the Bible and as such are blinded of their necessity.

I want to also note that the removal of these covenants and plain and precious writings was not the works of any religious church, sincere servant, or sincere translators. It appears to have taken place before that for -

"...after these plain and precious things were taken away it goeth forth unto all the nations of the Gentiles..." (Book of Mormon | 1 Nephi 13:29)

Nonetheless, with the coming forth of the Books of Mormon we receive many of the essential covenants and plain and precious things that assist us in qualifying for the celestial kingdom or to return and live worthily with The Father.