1 Nephi 13

As the angel shows Nephi the Bible, he explains -

"...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; wherefore, they are of great worth unto the Gentiles."  (Book of Mormon | 1 Nephi 13:23)

But that -

"...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.
26 And 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 13:24 - 26)

It appears then that sometime between the ministry and writing of the apostles and when monks made copies of their records, that wicked men got a hold of and removed precious and plain parts from the records.  Not only that, but also the  testimonies of covenants or ordinances that are essentials for salvation.  Covenants and ordinances are not required to receive the glory of terrestrial or telestial glories.

It is my personal opinion that Nephi is specifically mentioning the records by the hand of the apostles which is the New Testament.  I believe many plain and precious parts and covenants of the Old Testament had been somewhat purged.